the expression cos4x cos3x + sin4x sin3x is equivalent to

Question

which one of the following is the answer:
1) sinx
2) sin7x
3) cosx
4) cos7x

Answer 1

Look up the equation for cos(x-y) and it should be obvious what the answer is.

Think of x as 4x and y as 3x.

http://www.csgnetwork.com/trigtriformulatables.html

Answer 2

it is cosx
thanks

Answer 3

Seca=4x + 1/x than secb + tana what please tell me after 2days

Answer 4

fdgxdf