trig question (1 Viewer)

[alez]

ok i got this in a test and couldnt do it.
prove:
cos(p+q)cos(p-q)=cos'q-sin'p
' is squared there isnt squared button. dammit

 

[webby234]

cos(p+q) = cos(p)cos(q) - sin(p)sin(q)
cos(p-q) = cos(p)cos(q) + sin(p)sin(q)

Multiply them and you get:

[cos(p)cos(q)]² - [sin(p)sin(q)]²
= cos²(q)(1 - sin²(p)) - sin²(p)(1 - cos²(q))

The sin²(p)cos²(q) terms cancel and you get
cos²q - sin²p