已知cos(75°+x)=1/3,-180°

问题描述:

已知cos(75°+x)=1/3,-180°

∵-180°∴-195°即x-15°在第二或第三象限
∴cos(x-15°)又∵cos(75°+x)=cos[(x-15°)+90°]=-sin(x-15°)=1/3
∴sin(x-15°)=-1/3
则cos(x-15°)=-√[1-sin²(x-15°)]=-2√2/3

不懂可以追问!

cos(x-15°)=-sin(90°+(x-15°))=-sin(75°+x)
∵cos(75°+x)=1/3,-180°∴(-sin(75°+x))²=1-(cos(x-15°))²=8/9
∴-sin(75°+x)=(-2√2)/8
∴ cos(x-15°)=(-2√2)/3

因为cos(75°+x)=1/3,则cos(x-15°)=cos(75°+x-90°)=cos[90°-(75°+x)]=sin(75°+x),
又因为-180°