Worked solution
Repeat a logarithmic segment with the correct jump values
Let for , and for , with for all real .
(a) Rotate the region bounded by , the x-axis and through about the x-axis. Find the exact volume.
(b) Sketch for . Hence find .
[2014/ACJC/I/10(ii)]
CJC · Review problems · Question 14
Answer
; .
(a) Integrate the squared logarithm twice by parts
On the intended base-period region , . Two integrations by parts give . Hence . A source intermediate line drops the factor from , but its following evaluation and final answer use the correct antiderivative.
(b) Translate by multiples of two
On , ; on , . For the requested graph take for three complete periods and for the remaining segment . Each logarithmic top point is included, while the zero point immediately below it is open; the log starts are included. The last endpoint is .
(b) Method 1: use three full periods and a base partial period
(3 s.f.).
(b) Method 2: integrate the translated final segment
The last segment is on . Thus gives the same .
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Common mistakes
A periodic extension repeats both the logarithmic segment and the following zero interval.