cos(x-40)=cos(x+20)+cos(x-20)求tanx做出 来是根号3 代入进去也对的 但是这个答案是猜的

cos(x-40)=cos(x+20)+cos(x-20)cos(x-40)-cos(x+20)=cos(x-20)-2sin(x-10)sin(-30)=cos(x-20)sin(x-10)=cos(x-20)所以x-10与x-20互余,即 x-10+x-20=90+360nx=60+180ntanx=√3