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CCEA GCE Mathematics (2210) · Differentiation
Mini-Lesson

Differentiation

4 missions. Guess, learn, then check. Your place is saved.

  1. Mission 1Speed needs two moments. So what is your speed at one instant?
  2. Mission 2Fold a box from a 30 cm square card: what corner cut holds the most?
  3. Mission 3How do you differentiate e^(sin x), a function inside a function?
  4. Mission 4A balloon's radius grows 1 cm/s. Why does its volume grow ever faster?
Mission 1

Speed needs two moments. So what is your speed at one instant?

Make a guess: Speed needs two moments. So what is your speed at one instant?

Differentiation · first principles

Chords close in on the tangent

Chords approaching a tangent1234510152025Oxyy=x2hPQ1Q2tangent at P
  • The gradient of a curve at a point is the gradient of the tangent there.
  • A chord joins P(x, f(x)) to a nearby point Q(x+h, f(x+h)). Its gradient is
(f(x+h) − f(x))/h
  • As h → 0, Q slides towards P and the chord turns into the tangent. The tangent's gradient is the limit of the chord gradients.
  • We write it dy/dx or f'(x): the rate of change of y with respect to x.
Worked example Differentiation · first principles

Chord gradients on y = x²

Question
P(3, 9) lies on y = x². Find the gradients of the chords from P to the points with x = 3 + h for h = 1, 0.1 and 0.01, and hence estimate the gradient at P.
  1. 1
    Use the chord gradient ((3+h)² − 9)/h for each h.
  2. 2
    h = 1: (16 − 9)/1 = 7
  3. 3
    h = 0.1: (9.61 − 9)/0.1 = 6.1
  4. 4
    h = 0.01: (9.0601 − 9)/0.01 = 6.01
  5. 5
    The gradients approach 6. In fact ((3+h)² − 9)/h = 6 + h, which tends to 6 as h → 0.
Answer
the chord gradients are 7, 6.1 and 6.01; the gradient at P is 6.
⚠️ Watch out
putting h = 0 straight into ((3+h)² − 9)/h gives 0/0. Simplify first, then let h → 0.
Quick check
?On y = x³, what is the gradient of the chord from x = 2 to x = 2.1?
Differentiation · first principles

The derivative as a limit

  • Definition:
f'(x) = lim_(h → 0) (f(x+h) − f(x))/h
  • Method: expand f(x+h), subtract f(x), divide by h, then let h → 0.
  • For f(x) = x²: ((x+h)² − x²)/h = (2xh + h²)/h = 2x + h, which tends to 2x. So d/dx(x²) = 2x.
  • You may see δ x in place of h: dy/dx = lim_(δ x → 0) (δ y)/(δ x).
Calculate
?For f(x) = 2x² − 3x, show that (f(x+h) − f(x))/h = 4x + 2h − 3. Hence find the gradient of the curve y = f(x) at x = 5. Give an exact answer.
Calculate
?A ball rolls down a ramp so that it has travelled s = 0.6t² metres after t seconds. Show that its average speed from t = 3 to t = 3 + h is 3.6 + 0.6h m/s. Hence find its speed at the instant t = 3. Give an exact answer.
m/s
Mission check

?Speed needs two moments. So what is your speed at one instant?
Mission 2

Fold a box from a 30 cm square card: what corner cut holds the most?

Make a guess: Fold a box from a 30 cm square card: what corner cut holds the most?

Differentiation · stationary points

The power rule

  • For any rational n:
d/dx(axⁿ) = anxⁿ⁻¹
  • Differentiate a sum or difference term by term. A constant term differentiates to 0.
  • First rewrite as powers of x: √x = x^(1/2), 1/x² = x⁻², 3/(√x) = 3x^(−1/2). Expand brackets and split fractions over a single power.
  • Notation: dy/dx or f'(x). Differentiating again gives the second derivative d²y/dx² or f''(x).
Worked example Differentiation · stationary points

Rewriting before differentiating

Question
Given y = (x² + 6)/(√x), x > 0, find dy/dx and the gradient of the curve at x = 4.
  1. 1
    Split the fraction and write as powers: y = x^(3/2) + 6x^(−1/2)
  2. 2
    Multiply by the power, then reduce the power by 1: dy/dx = 3/2x^(1/2) − 3x^(−3/2)
  3. 3
    Substitute x = 4: 3/2(2) − 3 × 1/8 = 3 − 3/8
Answer
dy/dx = 3/2x^(1/2) − 3x^(−3/2); the gradient at x = 4 is 21/8 = 2.625.
⚠️ Watch out
you cannot differentiate the top and bottom separately. Rewrite as a sum of powers first.
Quick check
?Find dy/dx when y = 8/x³.
Differentiation · stationary points

Tangents and normals

Tangent and normal−11234567−224681012xyy=x2(3,9)tangent, gradient mnormal, gradient −1m
  • The gradient of the curve at x = a is m = f'(a). The tangent there has gradient m.
  • The normal is perpendicular to the tangent, so its gradient is −1/m.
  • Use y − y₁ = m(x − x₁) with the point (a, f(a)) on the curve.
  • Example: y = x² at (3, 9) has gradient 6, so the tangent is y − 9 = 6(x − 3), that is y = 6x − 9.
Calculate
?The curve C has equation y = x³ − 3x² + 2. The normal to C at the point where x = 3 meets the x-axis at Q. Find the x-coordinate of Q. Give an exact answer.
Differentiation · stationary points

Stationary points and their nature

Sign of the gradient1234−2−11234567xy(1,5)(3,1)(0,1)+−+
  • f is increasing where f'(x) > 0 and decreasing where f'(x) < 0.
  • At a stationary point dy/dx = 0. Solve this to find the x-coordinates.
  • Classify with the second derivative: d²y/dx² < 0 gives a maximum, d²y/dx² > 0 a minimum. If it is 0, check the sign of dy/dx either side.
  • Optimisation: write the quantity in terms of one variable, differentiate, set the derivative to 0, classify, then answer in context.
Worked example Differentiation · stationary points

Finding and classifying stationary points

Question
Find the stationary points of y = x³ − 6x² + 9x + 1 and determine their nature.
  1. 1
    Differentiate: dy/dx = 3x² − 12x + 9 = 3(x − 1)(x − 3)
  2. 2
    Set to zero: x = 1 or x = 3, giving y = 5 and y = 1
  3. 3
    Differentiate again: d²y/dx² = 6x − 12
  4. 4
    At x = 1: 6 − 12 = −6 < 0, a maximum. At x = 3: 18 − 12 = 6 > 0, a minimum.
Answer
(1, 5) is a maximum and (3, 1) is a minimum.
⚠️ Watch out
give both coordinates. Many students find x = 1 and x = 3 and stop, losing the y-values.
Sort it
?For f(x) = x³ − 12x, decide whether f is increasing or decreasing at each value of x.

Tap a statement, then tap the group it belongs in. Then press Check.

Calculate
?A closed cylindrical can has volume 500 cm³ and radius r cm. Show that its total surface area is A = 2π r² + 1000/r. Use calculus to find the value of r that makes A a minimum. Give your answer to 3 significant figures.
cm
Mission check

?Fold a box from a 30 cm square card: what corner cut holds the most?
Mission 3

How do you differentiate e^(sin x), a function inside a function?

Make a guess: How do you differentiate e^(sin x), a function inside a function?

Differentiation · chain, product and quotient rules

Standard derivatives (A2 only)

ydy/dx
e^kxke^kx
ln x (and ln kx)1/x
sin kxkcos kx
cos kx−ksin kx
tan kxksec² kx
  • Angles must be in radians. The trig results are false in degrees.
  • ln kx = ln k + ln x, so the constant k disappears when you differentiate.
Differentiation · chain, product and quotient rules

The chain rule (A2 only)

  • For a function of a function, y = f(u) where u = g(x):
dy/dx = dy/du × du/dx
  • In words: differentiate the outer function, leave the inside alone, then multiply by the derivative of the inside.
  • Rates multiply along the chain: if u changes 2 times as fast as x, and y 3 times as fast as u, then y changes 6 times as fast as x.
Worked example Differentiation · chain, product and quotient rules

Using the chain rule (A2 only)

Question
Differentiate y = (2x³ − 1)⁵.
  1. 1
    Name the inside: u = 2x³ − 1, so y = u⁵
  2. 2
    Differentiate each link: dy/du = 5u⁴ and du/dx = 6x²
  3. 3
    Multiply and put u back: dy/dx = 5(2x³ − 1)⁴ × 6x²
Answer
dy/dx = 30x²(2x³ − 1)⁴
⚠️ Watch out
5(2x³ − 1)⁴ alone forgets to multiply by the derivative of the inside. Expanding the bracket first works but wastes time.
Quick check
?Find dy/dx when y = e^3x².
Differentiation · chain, product and quotient rules

Product and quotient rules (A2 only)

  • Product rule: if y = uv, then dy/dx = udv/dx + vdu/dx.
  • Quotient rule: if y = u/v, then dy/dx = (vdu/dx − udv/dx)/v². The order on top matters.
  • The quotient rule on (sin x)/(cos x) gives (cos² x + sin² x)/(cos² x) = sec² x.
  • The chain or quotient rule gives d/dxsec x = sec xtan x, d/dxcosec x = −cosec xcot x, d/dxcot x = −cosec² x.
Worked example Differentiation · chain, product and quotient rules

Product rule and a stationary point (A2 only)

Question
Find the x-coordinates of the stationary points of y = x²e^3x.
  1. 1
    Set u = x² and v = e^3x: du/dx = 2x and dv/dx = 3e^3x
  2. 2
    Product rule: dy/dx = 3x²e^3x + 2xe^3x
  3. 3
    Factorise: dy/dx = xe^3x(3x + 2)
  4. 4
    Set to zero: e^3x > 0 for all x, so x = 0 or 3x + 2 = 0
Answer
x = 0 and x = −2/3
⚠️ Watch out
never divide by x to "cancel" it: that loses the solution x = 0. Factorise instead.
Calculate
?Find the gradient of the curve y = (2x − 1)/(x + 3) at the point where x = 1. Give your answer as a decimal.
Quick check
?Find dy/dx when y = ln 3x + sin 4x (x > 0, in radians).
Calculate
?The curve y = (x − 2)e^2x has one stationary point. Find its x-coordinate. Give your answer as a decimal.
Mission check

?How do you differentiate e^(sin x), a function inside a function?
Mission 4

A balloon's radius grows 1 cm/s. Why does its volume grow ever faster?

Make a guess: A balloon's radius grows 1 cm/s. Why does its volume grow ever faster?

Differentiation · rates, implicit and parametric

Connected rates of change (A2 only)

  • When two quantities are linked by a formula, their rates are linked by the chain rule:
dA/dt = dA/dr × dr/dt
  • Ripple: A = π r² and dr/dt = 2 cm/s. At r = 5: dA/dt = 2π r × 2 = 20π ≈ 62.8 cm²/s.
  • CCEA (A2 1) does not name connected rates of change: here it is an application of the chain rule you must know.
  • To construct a differential equation, turn words into a rate: "N grows at a rate proportional to N" becomes dN/dt = kN, with k > 0. "Decreases" needs a minus sign.
Calculate
?Water is poured into an inverted cone at a constant 6 cm³ per second. When the water is h cm deep, the surface radius is h/2 cm. Show that V = (π h³)/12. Find the rate at which the depth is rising when h = 4. Give your answer to 3 significant figures.
cm/s
Quick check
?In a simple model, the rate of decrease of the radius r of a mint is inversely proportional to the square of the radius. Which differential equation fits (k a positive constant, t time)?
Differentiation · rates, implicit and parametric

Implicit differentiation (A2 only)

  • In an equation like x² + xy + y² = 7, y is not the subject. Differentiate every term with respect to x.
  • A term in y needs the chain rule: d/dx(y³) = 3y²dy/dx.
  • A product needs the product rule: d/dx(xy) = xdy/dx + y.
  • Then collect the dy/dx terms on one side and factorise.
  • CCEA also asks for d²y/dx²: differentiate your dy/dx again, implicitly, and substitute.
Worked example Differentiation · rates, implicit and parametric

Gradient of an implicit curve (A2 only)

Question
Find the gradient of the curve x² + xy + y² = 7 at the point (1, 2).
  1. 1
    Check the point: 1 + 2 + 4 = 7, so (1, 2) is on the curve.
  2. 2
    Differentiate term by term: 2x + (xdy/dx + y) + 2ydy/dx = 0
  3. 3
    Collect: dy/dx(x + 2y) = −(2x + y), so dy/dx = −(2x + y)/(x + 2y)
  4. 4
    Substitute (1, 2): dy/dx = −4/5
Answer
the gradient at (1, 2) is −4/5 = −0.8.
⚠️ Watch out
differentiating xy as just dy/dx (no product rule) gives −2/5, a common lost mark.
Calculate
?For the circle x² + y² = 25, find the value of d²y/dx² at the point (3, 4). Give your answer as a decimal.
Differentiation · rates, implicit and parametric

Parametric differentiation (A2 only)

  • If x and y are both given in terms of a parameter t, use the chain rule:
dy/dx = dy/dt ÷ dx/dt
  • The answer is in terms of t. For a tangent or normal at a given t, find the point (x, y) and the gradient from the same t.
  • Second derivative: d²y/dx² = d/dt(dy/dx) ÷ dx/dt.
Worked example Differentiation · rates, implicit and parametric

Tangent to a parametric curve (A2 only)

A parametric curve with a loop12345−3−2−1123Oxyt=2
Question
A curve has parametric equations x = t² + 1, y = t³ − 3t. Find the equation of the tangent at the point where t = 2.
  1. 1
    Find the point: x = 5, y = 8 − 6 = 2
  2. 2
    Differentiate each: dx/dt = 2t = 4 and dy/dt = 3t² − 3 = 9 at t = 2
  3. 3
    Divide: dy/dx = 9/4
  4. 4
    Tangent: y − 2 = 9/4(x − 5), so 4y = 9x − 37
Answer
y = 9/4x − 37/4, or 9x − 4y − 37 = 0
⚠️ Watch out
dividing the other way, dx/dt ÷ dy/dt, gives 4/9: that is dx/dy.
Calculate
?For the curve x = t² + 1, y = t³ − 3t from the Example above, find the value of d²y/dx² at the point where t = 2. Give your answer as a decimal.
Mission check

?A balloon's radius grows 1 cm/s. Why does its volume grow ever faster?
Recap

Big ideas

  • The derivative dy/dx = f'(x) is the gradient of the tangent and the rate of change of y with respect to x.
  • It is the limit of chord gradients: f'(x) = lim_(h → 0) (f(x+h) − f(x))/h. Expand, subtract, divide by h, then let h → 0.
  • d/dx(axⁿ) = anxⁿ⁻¹ for rational n: rewrite roots and fractions as powers first.
  • Tangent gradient m = f'(a); normal gradient −1/m; then y − y₁ = m(x − x₁).
  • Stationary points solve dy/dx = 0; d²y/dx² < 0 means a maximum and > 0 a minimum.
  • Learn the standard derivatives of e^kx, ln x, sin kx, cos kx, tan kx, with x in radians.
  • Chain rule: dy/dx = dy/du × du/dx. Outer, keep the inside, times the inner derivative.
  • Product: udv/dx + vdu/dx. Quotient: (vdu/dx − udv/dx)/v².
  • Connected rates: dV/dt = dV/dr × dr/dt. Words like "proportional to" and "decreases" become a differential equation.
  • Implicit: differentiate every term with respect to x; yⁿ gives nyⁿ⁻¹dy/dx and xy needs the product rule.
  • Parametric: dy/dx = dy/dt ÷ dx/dt.
  • For d²y/dx², differentiate dy/dx with respect to t and divide by dx/dt again.
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Mini-lesson complete!

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You've worked through Differentiation for CCEA GCE Mathematics (2210). 🎉

  1. Mission 1Speed needs two moments. So what is your speed at one instant?
  2. Mission 2Fold a box from a 30 cm square card: what corner cut holds the most?
  3. Mission 3How do you differentiate e^(sin x), a function inside a function?
  4. Mission 4A balloon's radius grows 1 cm/s. Why does its volume grow ever faster?

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Specification points and sources for this lesson are listed at the bottom of the page.

Sources (2)

The specification points and sources behind this lesson.

  1. 1 CCEA GCE Mathematics (2018) specification, Unit AS 1 Differentiation
  2. 2 CCEA GCE Mathematics (2018) specification, Unit A2 1 Differentiation