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Asked by chris

find exact value cos(13pi/15)cos(-pi/5)-sin(13pi/15)sin(-pi/5)
14 years ago

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Answered by Reiny
do you see the pattern
cos(A + B) = cosAcosB - sinAsinB here ?

also remember that cos(-A) = cos (A)

give it a try
14 years ago
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find exact value cos(13pi/15)cos(-pi/5)-sin(13pi/15)sin(-pi/5)

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