若直線AX+BY-3=0與圓X^2+Y^2+4X-1=0切于點P(-1,2),則A*B的值為
圓X^2+Y^2+4X-1=0 == (x+2)^2 +y^2 = 5 圓心O(-2,0), 直線OP斜率=2直線L過點P(-1,2), 斜率=-1/直線OP斜率 = -1/2== 直線L方程: y-2 = (-1/2)(x+1) == x+2y-3=0== A = 1, B = 2== A*B = 2
