Sin5pi/8×sin7pi/8×sin7pi/4
1/2(сos(7π/8-5π/8)-cos(7π/8+5π/8))sin(2π-π/4)=1/2(cosπ/4-cos3π/2)*sin(-π/4)= =1/2(√2/2-0)*(-√2/2)=-1/2*(√2/2)²=-1/2*1/2=-1/4