1.50
x=0亦然
2.b-c = (a-c )-(a-b)代入左端
((a-c)-(a-b))^2 = -4(a-c)(a-b) 有变号
右边移到左边:((a-c)+(a-b))^2 = 0
a-c + a-b = 0
a = b+c