5cos2a если cosa = -0,9

Question

5cos2a если cosa = -0,9

Answer 1

cosa=-0.5

sin^2(a)=1-cos^2(a)=1-0.25=0.75

5cos2a=5(cos^2(a)-sin^2(a))=5*(0.25-0.75)=5*(-0.5)=-2.5