cos(π/4-a)=2√2·cos2acos(π/4)cosa+sin(π/4)sina=2√2(cos²a-sin²a)(√2/2)(cosa+sina)=2√2·(cosa+sina)(cosa-sina)约去cosa+sina,易得cosa-sina=1/4平方,得1-2sinacosa=1/16所以sin2a=15/16...