trig prob please help!!?

Question

If f(x) = cos x and f(a) = 1/4 find the exact value of:

a) f(-a)

b) f(a)+f(a+2pi)+f(a-2pi)

Answer 1

cos(x) is an even function. This means:
cos(x) = cos(-x)

Therefore
cos(-a) = cos(a) = 1/4

cos(x) has a period of 2π. This means:
cos(x) = cos(x + 2πk)
where k is an integer

f(a) + f(a + 2π) + f(a - 2π) = 3f(a) = 3*(1/4) = 3/4

Answer 2

a)
f(x) = cos(x)
f(x) is and even function
f(a) = 1/4
f(-a) = 1/4

b)
f(x) is a periodic function with a period of 2pi
f(a) + f(a+2pi) + f(a-2pi)
= f(a) + f(a) + f(a)
= 3*f(a)
= 3/4