重点在于 你要能看出来xcosΘ+ysinΘ=2是半圆x^2+y^2=4(y>=0)的切线
xcosΘ+ysinΘ=2
即
x*(2cosΘ)+y(2sinΘ)=4
x0= r*cosΘ = 2cosΘ y0 = r*sinΘ = 2sinΘ
x*x0 + y*y0 = 4 = r^2
故xcosΘ+ysinΘ=2是半圆x^2+y^2=4(y>=0)的切线
看到这里就请画个图吧
两曲线公切线有两条:
xcos(π/4)+ycos(π/4)=2
xcos(3π/4)+ycos(3π/4)=2
所以D