化简求值：3（a+1)²-5（a+1)(a-1)+2(a-1)²,其中a=-1/21.已知（x+y）²=16,（x-y）²=4,求xy的值2.已知x+y=5,x-y=1,求xy的值

Question

化简求值：3（a+1)²-5（a+1)(a-1)+2(a-1)²,其中a=-1/2
1.已知（x+y）²=16,（x-y）²=4,求xy的值
2.已知x+y=5,x-y=1,求xy的值

Answer 1

3（a+1)²-5（a+1)(a-1)+2(a-1)²
=(3(a+1)-2(a-1))((a+1)-(a-1))
=(3a+3-2a+2)(a+1-a+1)
=2(a+5)
=2a+10
=2*(-1/2)+10
=9
1.已知（x+y）²=16,（x-y）²=4,求xy的值
x²+2xy+y²=16
x²-2xy+y²=4
所以4xy=12
得xy=3
2.已知x+y=5,x-y=1,求xy的值
(x+y)²=5²,即x²+2xy+y²=25
(x-y)²=1²,即x²-2xy+y²=1
得4xy=24
即xy=6