Question

calcule cos 7pi/8 . cos pi/8

Answer 1

cosa.cosb=½[cos(a+b) +cos(a-b)]

cos(7π/8). cos(π/8)=½(cos([7π+π]/8)+cos([7π-π]/8))

=½(cos(8π/8)+cos(6π/8))

=½(cosπ+cos(3π/4))

=½(-1 - √2/2)