Question

Integration of cos6x

Answer 1

=sin6x/6
..............

Answer 2

u = 6x
du = 6 dx

cos (6x) = cos (u)

integrate: d/dx of cos (6x) = *subsititute* d/(du/6) of cos (u)
1/6 sin (u) = 1/6 sin(6x) + C
(if you want to simplify it further) = sin(6x) + C* where C* = 6C