4x²+4bx+7b=0有两个相等实数根
(1)则△=(4b)²-4x4x(7b)=16b²-16x7b=0 b²-7b=0 b=0或b=7
(2)y²-(b-2)y+4=0
①当b=0时,y²+2y+4=0 △=2²-4x4=-12