2022 May/June · Paper 22 · Q8 (a)5 marksStationary points
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Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Tangents and normals · Core · Paper 1 style · no calculator · 4 marks
(a)Find the equation of the tangent to the curve y = x³ − 4x + 1 at the point where x = 2.[4]
Show answer
(a) y = 8x − 15
dy/dx = 3x² − 4 B1; at x = 2, gradient 8 B1, y = 1 B1; y − 1 = 8(x − 2), y = 8x − 15 A1
Q2Tangents and normals · Core · Paper 1 style · no calculator · 4 marks
(a)Find the equation of the tangent to the curve y = e^(2x) − 3x at the point where the curve crosses the y-axis.[3]
(b)Write down the equation of the normal at the same point.[1]
Show answer
(a) y = 1 − x
dy/dx = 2e^(2x) − 3 B1; at x = 0, y = 1 and gradient −1 M1; y = 1 − x A1
(b) y = x + 1
gradient 1, through (0, 1) B1
Q3Tangents and normals · Core · Paper 1 style · no calculator · 5 marks
(a)Find the equation of the normal to the curve y = ln(3x − 2) at the point where x = 1.[4]
(b)Find the coordinates of the point where the normal meets the y-axis.[1]
Show answer
(a) y = −⅓(x − 1), i.e. x + 3y = 1
dy/dx = 3/(3x − 2) M1 A1; at x = 1, y = 0, gradient 3 B1; normal gradient −⅓, x + 3y = 1 A1
(b) (0, ⅓)
B1
Q4Tangents and normals · Standard · Paper 1 style · no calculator · 6 marks
(a)Find the equation of the normal to the curve y = 2x² − 3x + 5 at the point (1, 4).[3]
(b)Find the coordinates of the point where this normal meets the curve again.[3]
Show answer
(a) y = −x + 5
dy/dx = 4x − 3 = 1 at x = 1 M1; normal gradient −1 M1; y = −x + 5 A1
Q5Tangents and normals · Standard · Paper 1 style · no calculator · 4 marks
(a)Show that the tangent to the curve y = x sin x at the point where x = π/2 passes through the origin.[4]
Show answer
(a) Tangent is y = x, which passes through (0, 0)
dy/dx = sin x + x cos x M1 A1; at x = π/2, y = π/2, gradient 1 A1; y − π/2 = x − π/2 ⇒ y = x A1
Q6Tangents and normals · Standard · Paper 2 style · calculator · 6 marks
(a)Find the equation of the normal to the curve y = (x² + 3)/(x − 1) at the point where x = 2.[5]
(b)Find the x-coordinate of the point where this normal meets the x-axis.[1]
Show answer
(a) 3y = x + 19
quotient rule: dy/dx = [2x(x − 1) − (x² + 3)]/(x − 1)² M1 A1; at x = 2, y = 7, gradient −3 A1; normal gradient ⅓ M1; 3y = x + 19 A1
(b) x = −19
B1
Q7Tangents and normals · Challenge · Paper 1 style · no calculator · 8 marks
(a)Find the equations of the tangent and the normal to the curve y = √(4x + 1) at the point where x = 2.[5]
(b)The tangent and the normal meet the x-axis at A and B respectively. Find the area of the triangle formed by A, B and the point (2, 3).[3]
Show answer
(a) tangent 3y = 2x + 5; normal 2y = −3x + 12
dy/dx = 2(4x + 1)^(−½) M1 A1; at x = 2, y = 3, gradient 2/3 A1; tangent y − 3 = ⅔(x − 2) A1; normal y − 3 = −3/2(x − 2) A1
(b) 39/4 (9.75) square units
A(−2.5, 0), B(4, 0) B1 B1; area = ½ × 6.5 × 3 = 9.75 B1
Q8Tangents and normals · Challenge · Paper 1 style · no calculator · 6 marks
(a)The line y = 3x − 4 is a tangent to the curve y = x² + kx, where k is a constant. Find the possible values of k and, for each value, the coordinates of the point of contact.[6]
Show answer
(a) k = −1, contact (2, 2); k = 7, contact (−2, −10)
2x + k = 3 M1; x² + kx = 3x − 4 M1; substitute k = 3 − 2x: −x² = −4 M1; x = ±2 A1; k = −1 at (2, 2) A1; k = 7 at (−2, −10) A1
Q9Derivatives (chain, product, quotient) · Core · Paper 1 style · no calculator · 4 marks
(a)Differentiate (2x − 5)⁶ with respect to x.[2]
(b)Differentiate x²e^(3x) with respect to x.[2]
Show answer
(a) 12(2x − 5)⁵
6(2x − 5)⁵ × 2 M1 A1
(b) e^(3x)(2x + 3x²)
product rule M1; 2xe^(3x) + 3x²e^(3x) A1
Q10Derivatives (chain, product, quotient) · Core · Paper 1 style · no calculator · 5 marks
(a)Find d/dx (sin 2x ÷ x).[3]
(b)Find d/dx (tan 3x).[2]
Show answer
(a) (2x cos 2x − sin 2x)/x²
quotient rule M1; 2 cos 2x seen B1; correct A1
(b) 3 sec² 3x
sec² 3x M1; × 3 A1
Q11Derivatives (chain, product, quotient) · Core · Paper 1 style · no calculator · 4 marks
(a)Given that y = e^(2x) cos x, find dy/dx.[3]
(b)Find the exact value of dy/dx when x = π/2.[1]
Show answer
(a) e^(2x)(2 cos x − sin x)
product rule M1; 2e^(2x) cos x B1; − e^(2x) sin x A1
(b) −e^π
e^π(0 − 1) B1
Q12Derivatives (chain, product, quotient) · Standard · Paper 1 style · no calculator · 4 marks
5x² + 6x − 1 = 0 M1; x = (−6 ± √56)/10 A1; = (−3 ± √14)/5, both > −1.5 so both valid A1
Q16Derivatives (chain, product, quotient) · Challenge · Paper 1 style · no calculator · 5 marks
(a)Given that y = sin³(2x), find dy/dx.[3]
(b)Find the exact value of dy/dx when x = π/12.[2]
Show answer
(a) 6 sin²(2x) cos(2x)
chain rule: 3 sin²2x × cos 2x × 2 M1 M1 A1
(b) 3√3/4
6 × (½)² × (√3/2) M1 = 3√3/4 A1
Q17Small changes and rates of change · Core · Paper 2 style · calculator · 4 marks
(a)A spherical balloon is being inflated so that its radius increases at a constant rate of 0.2 cm per second. Find the rate of increase of the volume of the balloon at the instant when the radius is 5 cm. Give your answer correct to 3 significant figures.[4]
Q18Small changes and rates of change · Core · Paper 1 style · no calculator · 3 marks
(a)Given that y = 4/x², find dy/dx.[1]
(b)Hence find, in terms of h, the approximate change in y when x increases from 2 to 2 + h, where h is small.[2]
Show answer
(a) −8/x³
B1
(b) −h (a decrease of h)
dy/dx at x = 2 is −1 M1; δy ≈ −1 × h A1
Q19Small changes and rates of change · Standard · Paper 2 style · calculator · 4 marks
(a)An oil spill in the Arabian Sea forms a circle whose area is increasing at a constant rate of 50 m² per second. Find the rate at which the radius is increasing at the instant when the radius is 20 m. Give your answer correct to 3 significant figures.[4]
Q20Small changes and rates of change · Standard · Paper 2 style · calculator · 5 marks
A container is in the shape of a right circular cone with its vertex at the bottom and its axis vertical. The radius of the top of the cone is half its height.
(a)Show that, when the depth of water is h cm, the volume of water is V = πh³/12 cm³.[2]
(b)Water is poured into the container at a constant rate of 3 cm³ per second. Find the rate at which the depth is increasing when h = 6. Give your answer correct to 3 significant figures.[3]
Show answer
(a) shown
radius of surface = h/2 B1; V = ⅓π(h/2)²h = πh³/12 B1
Q22Small changes and rates of change · Challenge · Paper 2 style · calculator · 5 marks
(a)The surface area of a cube is increasing at a constant rate of 6 cm² per second. Find the rate at which the length of each edge is increasing when the edge is 4 cm long.[3]
(b)Hence find the rate at which the volume of the cube is increasing at this instant.[2]
Q28Practical maxima and minima · Core · Paper 2 style · calculator · 5 marks
A farmer in Punjab uses 120 m of fencing to enclose a rectangular field next to a straight river. The river forms one side, so fencing is needed on the other three sides only. The two sides perpendicular to the river each have length x m.
(a)Show that the area A m² of the field is given by A = 120x − 2x².[1]
(b)Find the value of x for which A is a maximum, and find this maximum area.[4]
Show answer
(a) shown
length parallel to river = 120 − 2x, so A = x(120 − 2x) B1
(b) x = 30, A = 1800 m²
dA/dx = 120 − 4x M1; = 0, x = 30 A1; A = 1800 A1; d²A/dx² = −4 < 0, maximum B1
Q29Practical maxima and minima · Standard · Paper 2 style · calculator · 7 marks
A closed box is a cuboid with a base x cm by 2x cm and height h cm. Its volume is 576 cm³.
(a)Show that the total surface area, A cm², of the box is given by A = 4x² + 1728/x.[3]
(b)Find the value of x for which A is a minimum, and find this minimum surface area.[4]
Show answer
(a) shown
2x²h = 576 so h = 288/x² B1; A = 2(2x² + xh + 2xh) = 4x² + 6xh M1; = 4x² + 1728/x A1