2cos A cos B=cos(A+B)+cos(A-B)2cosAcosB=cos(A+B)+cos(A−B)
cosAcos B=1/2 (cos(A+B)+cos(A-B))cosAcosB=12(cos(A+B)+cos(A−B))
A=(15pi)/8, B=(5pi)/8A=15π8,B=5π8
=>cos((15pi)/8)cos ((5pi)/8)=1/2 (cos((15pi)/8+(5pi)/8)+cos((15pi)/8-(5pi)/8))⇒cos(15π8)cos(5π8)=12(cos(15π8+5π8)+cos(15π8−5π8))
=1/2 (cos((20pi)/8)+cos((10pi)/8)) =12(cos(20π8)+cos(10π8))
=1/2 cos((5pi)/2)+1/2 cos((5pi)/4) =0+ -sqrt2/2=-sqrt2/2 =12cos(5π2)+12cos(5π4)=0+−√22=−√22
cos((15pi)/8)cos ((5pi)/8)=1/2 cos((5pi)/2)+1/2 cos((5pi)/4)=-sqrt2/2cos(15π8)cos(5π8)=12cos(5π2)+12cos(5π4)=−√22