sin(Π/4 -x)=5/13 √2/2(cosx-sinx)=5/13x∈(0,Π/4),cos(Π/4 -x)=12/13 √2/2(cosx+sinx)=12/13 相乘,得1/2(cos^2x-sin^2x)=60/169cos2x=120/169cos(Π/4 +x)=sin(Π/4 -x)=5/13(cos2x)/[cos(Π/4 +x)]=(120/169)...cos(Π/4 +x)=sin(Π/4 -x)=5/13 怎么得到的？诱导公式 cos(π/2-x)=sinx 即两个角互余(π/2-x)+x=π/2 则两个角的余函数值相等 (π/4 +x)+(π/4 -x)=π/2所以cos(Π/4 +x)=sin(Π/4 -x)