SOLUTION: Use product-to-sum identity to rewrite the expression cos 40° cos 20° Thank you

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Question 1101605: Use product-to-sum identity to rewrite the expression
cos 40° cos 20°
Thank you
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
cos(x)cos(y)=(1/2)​​ [cos(x−y)+cos(x+y)]
cos 40*cos 20=(1/2)[(cos 20)+cos (60)]
Check
cos 40*cos 20=0.7198
cos 20+cos 60=1.44
half of that is 0.72 (0.7198 with less rounding).