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

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Answer 1 · 2016-02-19T11:08:32

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