Question

steps to find the nth derivative of y = cos5x.cos3x

Answer 1

Is that dot used to represent multiplication?
\[y=\cos5x \times \cos3x\]

Answer 2

f(x) = g(x) y(x)
f'(x) = g(x)y'(x) + f(x) g'(x)

Answer 3

You can use wolfram
http://www.wolframalpha.com/input/?i=+derivative+of+y+%3D+cos5x%28cos3x%29

Answer 4

They're steps too

Answer 5

mul wiht 2and divide with 2
\[{\cos(5x+3x)+\cos(5x-2x)}/2\]
or
\[(\cos7x+\cos3x)/2\]
then
apply nth derivative