cos3(pi)/5 cos3(pi)/20 - sin3(pi)/5 sin3(pi)/20

cos(A+B) = cos(A)cos(B) - sin(A)sin(B) so 

cos(3pi/5)cos(3pi/20) - sin(3pi/5)sin(2pi/20) = cos(3pi/5 + 3pi/20) or cos(15pi/20) or cos(3pi/4) or cos(135°)

cos(135°) = -√2/2

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\mathtt{135}}^\circ\right)} = -{\mathtt{0.707\: \!106\: \!781\: \!187}}$$