已知x+y=根号2sin(α+π/4),x-y=根号2sin(α-π/4),求证x²+y²=1
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已知x+y=根号2sin(α+π/4),x-y=根号2sin(α-π/4),求证x²+y²=1
![已知x+y=根号2sin(α+π/4),x-y=根号2sin(α-π/4),求证x²+y²=1](/uploads/image/z/17291008-64-8.jpg?t=%E5%B7%B2%E7%9F%A5x%2By%3D%E6%A0%B9%E5%8F%B72sin%28%CE%B1%2B%CF%80%2F4%29%2Cx-y%3D%E6%A0%B9%E5%8F%B72sin%28%CE%B1-%CF%80%2F4%29%2C%E6%B1%82%E8%AF%81x%26sup2%3B%2By%26sup2%3B%3D1)
联立方程
x+y=根号2sin(α+π/4)
x-y=根号2sin(α-π/4)
解得:x=(根号2/2)*(sin(α+π/4)+sin(α-π/4)),y=(根号2/2)*(sin(α+π/4)-sin(α-π/4));
由三角和差化积公式得:sin(α+π/4)=sinα*cos(π/4)+cosα*sin(π/4),sin(α-π/4)=sinα*cos(π/4)-cosα*sin(π/4);
故sin(α+π/4)+sin(α-π/4)=2*sinα*cos(π/4)=根号2*sinα;
sin(α+π/4)-sin(α-π/4)=2*cosα*sin(π/4)=根号2*cosα;
代入x,y可得:x=sinα,y=cosα;
所以,x²+y²=sin²α+cos²α=1.
x+y=根号2sin(α+π/4)
x-y=根号2sin(α-π/4)
解得:x=(根号2/2)*(sin(α+π/4)+sin(α-π/4)),y=(根号2/2)*(sin(α+π/4)-sin(α-π/4));
由三角和差化积公式得:sin(α+π/4)=sinα*cos(π/4)+cosα*sin(π/4),sin(α-π/4)=sinα*cos(π/4)-cosα*sin(π/4);
故sin(α+π/4)+sin(α-π/4)=2*sinα*cos(π/4)=根号2*sinα;
sin(α+π/4)-sin(α-π/4)=2*cosα*sin(π/4)=根号2*cosα;
代入x,y可得:x=sinα,y=cosα;
所以,x²+y²=sin²α+cos²α=1.
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