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prove that sin(n+1)x.sin(x+2)+cos(n+1)x.cos(n+2)x=cosx |
| cos(A - B) = cosAcosB + sinAsinB sin(n+1)x.sin(n+2)x+cos(n+1)x.cos(n+2)x =cos(nx+x-nx-2x) =cos(-x) =cosx cos(A - B) = cosAcosB + sinAsinB cos(A - B) = cosAcosB + sinAsinB |
