5π/ 8 = (π - 3π/8)
7π/8 = ( π - π/8)
7π/4 = (2π - π/4)
1) = Sin 3π/8 Sin π/8 (-Sin π/4)= 1/2( Cosπ/4 - Cos π/2)· (- √2/2)=
=1/2·√2/2·(-√2/2) = - 1/4
2) 39π/4 = (10π - π/4)
= (Сos 3π/8 - Cos π/8) ( Сos 3π/8 + Cos π/8) Сos π/4 =
=- 2Sin π/4 Sin π/8·2 Cosπ/4 Cos π/8· √2/2 =
=-2·√2/2 Sin π/8·2 √2/2 ·Cos π/8 ·√2/2 = -√2/2·Sin π/4 = - √2/2· √2/2 = -1/2