设直线x/a+y/b=1过M(cos a,sin a),则( )A.a^2+b^2B.a^2+b^2>=1C.1/a^2+1/b^2D.1/a^2+1/b^2>=1

问题描述:

设直线x/a+y/b=1过M(cos a,sin a),则( )
A.a^2+b^2B.a^2+b^2>=1
C.1/a^2+1/b^2D.1/a^2+1/b^2>=1

选D
过M(cos a,sin a),说明直线与圆相切或相交,其充要条件是:圆心到直线距离d小于等于半径R
d=1/squr(1/a^2+1/b^2)解得1/a^2+1/b^2>=1