The essence of this problem is to find the inequality that A and B satisfy when x/a+y/b= 1 and X 2+Y 2 =1have something in common.

Under the condition that the distance from the center of the circle to the straight line is the root number, the distance from the point to the straight line is1/(1/a 2+1/b 2).

According to the positional relationship between a straight line and a circle, the root number is1/(1/a2+1/b2) ≤1.

So1/a 2+1/b 2 ≥1.