Question

Need help!!
2. Give that cos (x+y) = cos x cos y – sin x sin y, use this to express cos 2x in terms of cos x

Answer 1

so replace y with x

so you have

cos(x + x) = cos(x)cos(x) - sin(x)sin(x)

and simplify from there.

Answer 2

so the left side of the equation becomes

cos(2x)

Answer 3

why do you do that

Answer 4

now you can simplify the right side

Answer 5

I mean why do you change the y into x

Answer 6

that way when you replace y with x

cos(x + y) = cos(x + x) which is cos(2x)