分子那里变成加25 了啊
因为tanα=sinα/cosα = 5/12, 所以有cosα=(12sinα)/5,
又有(cosα)^2+(sinα)^2=1, 将cosα=(12sinα)/5代入后就有
(12sinα/5)^2 + (sinα)^2=1, 即144 (sinα)^2/25 + (sinα)^2=(144+25)/25 (sinα)^2=1
通分合并同类项