Let α and β be first quadrant angles with cos ( α ) = √ 2 / 8 and sin ( β ) = √ 7 / 10 . Find cos ( α − β )

Question

How do I solve this problem? I know I have to use Sum Difference Identities and use cos(a-b) = cos(a)cos(s)-sin(a)sin(b) but how do I get those values?

Answer 1

If you know cos(α) you can get sin(α) from the identity

sin²(α) + cos²(α) = 1

Similarly you can get cos(β) from sin(β).