How do you express cos( (15 pi)/ 8 ) * cos (( 23 pi) /12 ) without using products of trigonometric functions?

1 Answer
Feb 19, 2016

Well we know;

2cosa cos b = cos( a+ b) + cos (a-b)

cosa cos b =( cos( a+ b) + cos (a-b))/2

Now Substitute

=>cos15pi/8 cos23pi/12 =( cos( 15pi/8 + 23pi/12) + cos (15pi/8-23pi/12))/2

=>cos15pi/8 cos23pi/12 =( cos(91/24 pi ) + cos (-1/244 pi))/2

There we have used sums to express the Function