Question

1. Cos(2arccos4/5)
2.cos(pi-2arccos4/5)

Answer 1

1) =2*cos(arccos(4/5))-1=2*(4/5)^2-1=32/25-1=7/25  (по формуле cos^2 x=(1+cos2x)/2
2) =-cos 2arccos(4/5)=-7/25

Answer 2

1. Cos(2arccos4/5)=2cos²(arrcos4/5)-1=2*4/5*4/5-1=32/25-1=7/25
2.cos(pi-2arccos4/5)=-cos(2arccos4/5)=-7/25