A)15cosx=3cosx*5sinx
15cosx/cosx=3cosx/cosx*5sinx/cosx
15=3*5tgx
5tgx=5
tgx=1
x=П/4+Пn
б)5П<П/4+Пn<13П/2
5П-П/4<П/4+Пn-П/4<13P/2-П/4
10P/4-P/4<Pn<26P/4-P/4
9P/4<Pn<25P/4
2,25<n<6,25
n=3,4,5,6
64х²-144+81 = (8х)²- 2*8*9+9² = (8х - 9)²= (8х - 9)(8х - 9)