С0s^2 (a- п/4) - С0s^2 (a+ п/4)

(cos(a-π/4)-cos(a+π/4))(cos(a-π/4)+cos(a+π/4))==-2sin(-π/4)sina*2cos(-π/4)cosa=2*√2/2sina*2*√2/2cosa==2sinacosa=sin2a