Destiny of Pi 2 Report post Posted April 19, 2017 Hi guys, I have a question on Calculus and it has two sub-questions. It would be great if any of you could help me solve this. Sub-question 1, do I equate the equation to y equals something and then solve? And I have no idea on how to solve the second sub-question. Share this post Link to post Share on other sites

kw0573 1,113 Report post Posted April 22, 2017 (edited) sub q 1: know what formula to use for revolution around x axis (int of pi y^2 dx from x=a to x=b) and express y^2 in terms of x. sub q 2: this is hl, it seems. use V around y-axis (int of pi x^2 dy from y=a to y=b) however note there is a large hole in the middle of the resulting solid. Give it a shot! Now that you know the formula, you should be able to derive the integral using intersection of graphs and inverse functions. Edited April 22, 2017 by kw0573 Share this post Link to post Share on other sites

Destiny of Pi 2 Report post Posted April 28, 2017 On 2017/4/22 at 0:41 AM, kw0573 said: sub q 1: know what formula to use for revolution around x axis (int of pi y^2 dx from x=a to x=b) and express y^2 in terms of x. sub q 2: this is hl, it seems. use V around y-axis (int of pi x^2 dy from y=a to y=b) however note there is a large hole in the middle of the resulting solid. Give it a shot! Now that you know the formula, you should be able to derive the integral using intersection of graphs and inverse functions. This is what I got for sub 1: int (x^2-9) dy 1/3 x^3 - 9x and the answer if 110. for sub 2, still don't know how to work out the hole in the middle, this is what I got so far: int (9+y^2) dx - int sqrt(9+y^2) dx I wonder what should I do next. Share this post Link to post Share on other sites

kw0573 1,113 Report post Posted April 28, 2017 (edited) 1) Leave in exact form (n pi). I got a different answer, 44/3 pi, which is about 46. 2) This is HL. You need to integrate over dy (not dx), see formula i gave in previous response. May this diagram help Edited May 2, 2017 by kw0573 Share this post Link to post Share on other sites