a(a-1)+(b-a的2次方)=-7
a²-a+b-a²=-7
a-b=7
(a²+b²)/2-ab
=1/2(a²-2ab+b²)
=1/2(a-b)²
=1/2×7²
=24.5-ab呢？括号中，配成（a-b)²