The question is:
i) Find the maximum and minimum values.
ii) the smallest non-negative value of x for which this occurs.
12cos(a)-9sin(a)
I think it should be changed into the form of Rcos(a+x) and it should be 15cos(a+36.87), and I get the answer i)+15 / -15 ii)323.13 (360-36.87) / 143.13 (180-36.87).
But the answer given by the book is " i)15, -15 ii)306.87, 143.13 "
I'm really confused by that answer..Am I wrong?
BTW, I'm self studying A-level further pure mathematics, but the book(written by BRIAN and MARK GAULTER published by Oxford university press) I get seems not very helpful.
so I truly hope someone can recommend some books/websites for self learning.