the expression cos4x cos3x + sin4x sin3x is equivalent to

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

it is cosx

thanks

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

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

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To find the equivalent expression for cos(4x) cos(3x) + sin(4x) sin(3x), we can use the trigonometric identity for the product of two cosines and two sines.

The identity is:
cos(a - b) = cos(a) cos(b) + sin(a) sin(b)

In our expression, we can rewrite cos(4x) cos(3x) + sin(4x) sin(3x) as cos(4x - 3x).

Simplifying the expression, we get:
cos(4x - 3x) = cos(x)

Therefore, the equivalent expression is cos(x), which corresponds to option 3) cosx.