# How do you express #cos(pi/ 2 ) * cos (( 17 pi) / 12 ) # without using products of trigonometric functions?

##### 1 Answer

Jan 5, 2016

#### Explanation:

Use the rule:

#cos(a)cos(b)=1/2(cos(a+b)+cos(a-b))#

Thus,

#cos(pi/2)cos((17pi)/12)=1/2(cos(pi/2+(17pi)/12)+cos(pi/2-(17pi)/12))#

#=1/2(cos((23pi)/12)+cos((-11pi)/12))#

This could continue to be simplified using half angle formulas, but this answer is fine as is given the parameters ("without using products of trigonometric functions").