求二元函数最值的常用方法有:化二元函数为一元函数,基本不等式法,几何意义法。例题:已知圆O:x2+y2=4和定点A(2,2),由圆O外一点P(a,b)向圆O引切线PQ,切点为Q,且PQ=PA成立,求PQ的最小值。 The most commonly used method for finding the best value of a binary function is: The Binary Function is a unary function, the basic inequality method, the geometric meaning method. Example: Knowing the circle O: x2 + y2 = 4 and the fixed point A (2,2), the line PQ is drawn from the point O (a, b) outside the circle O to the circle O, the point of contact is Q, and PQ = PA holds, Find the minimum PQ.