1.将（sinα+cosα)^2和（sinα-cosα)^2拆开后化简
2.tanα写成sinα/cosα,在与sinαcosα通分
3.化简
用到的公式：(a+b)^2=a^2+b^2+2ab
sin^2α+cos^2α=1
sinα/cosα=tanα
cosα/sinα=cotα
原式=[sin^2a+cos^2a+2sinacosa-sin^2a-cos^2a+2sinacosa]/tana-sinacosa]
=4sinacosa/[(sina/cosa)-sinacosa]
=4sinacosa/sina(1-cos^2 a)/cosa
=4sinacos^2a/sina*sin^2 a
=4sinacos^2a/sin^3 a
=4cos^2a