[中學] 三角函數
學校段考題
若sinX+cosY=2/3,cosX+sinY=4/3,
求sin(X+Y)=________;cos(X-Y)=________。
[答案]:(1)1/9 (2)4/5
想問第二小題
我算到最後算出 4/5 or -4/5
但是不知道怎麼取值
怎麼下手比較好
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