证明：∵sinθ sinα cosθ成等差数列∴2sina=sinθ+cosθ,∵sinθ sinβ cosθ为等比数列,∴sin^2β=sincθosθ,∴左边=2cos2a=2[1-2(sina)^2]=2-4(sina)^2=2-(sinθ+cosθ)^2=2-(1+2sinθcosθ)=1-2sinθcosθ=1-2...