Question
find exact value cos(13pi/15)cos(-pi/5)-sin(13pi/15)sin(-pi/5)
Answers
do you see the pattern
cos(A + B) = cosAcosB - sinAsinB here ?
also remember that cos(-A) = cos (A)
give it a try
cos(A + B) = cosAcosB - sinAsinB here ?
also remember that cos(-A) = cos (A)
give it a try