А)cos15°cos75°=cos15°cos(90°-15°)=cos15°sin15°=1/2*2sin15°cos15°=1/2*sin30°=1/2*1/2=1/4=0,25
б)(cosπ/8+sinπ/8)*(cos³π/8-sin³π/8)=(cosπ/8+sinπ/8)*(cosπ/8-sinπ/8)*(cos²π/8+cosπ/8*sinπ/8+sin²π/8)=(cos²π/8-sin²π/8)*(1+1/2*2sinπ/8cosπ/8)=cosπ/4*(1+1/2*sinπ/4)=
=√2/2(1+√2/4)=√2/2+1/4=(2√2+1)/4