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3.8.5 Integration, Long Questions (Question 9)


Question 9:
Diagram 4 shows part of a curve x = y2 + 2. The gradient of a straight line QR is –1.

Find
(a) the equation of PQ,   [2 marks]
(b) the area of shaded region, [4 marks]
(c) the volume of revolution, in terms of π, when the shaded region is rotated through 360o about the y-axis. [4 marks]



Solution:
(a)
– 2 = –1 (x – 6)
= –x + 6 + 2
y = –x + 8

(b)
= –x + 8, at y-axis, x = 0
= 8
Point R = (0, 8)
Area of shaded region
= Area under the curve + Area of triangle
=02xdy+12(82)(6)=02(y2+2)dy+12(6)(6)=[y33+2y]02+18=[233+2(2)0]+18=203+18=2423unit2

(c)
Volume of revolution
= Volume under the curve + Volume of cone
=π02x2dy+13πr2h=π02(y2+2)2dy+13π(6)2(6)=π02(y4+4y2+4)dy+72π=π[y55+4y33+4y]02+72π=π[255+4(2)33+4(2)0]+72π=π[325+323+8]+72π=37615π+72π=97115πunit3


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