sina+cosa=√2(sina/√2+cosa/√2)=√2(sinacosπ/4+cosasinπ/4)=√2sin(a+π/4)第二步怎么到第三步？的sin(π/4)=cos(π/4)=1/√2√2(sinacosπ/4+cosasinπ/4)=√2sin(a+π/4) 是为什么三角函数的和差公式呀。sin(a+b)=sinacosb+sinbcosa.