出错了吧?中间漏了个2?sin^2a+sin^2β-sin^2a*sin^2β+cos^2a*cos^2β=sin^2a-sin^2a*sin^2β+cos^2a*cos^2β+sin^2β=sin^2a*cos^2β+cos^2a*cos^2β+sin^2β=cos^2β*（sin^2a+cos^2a）+sin^2β=cos^2β+sin^2β=1...