1.已知x^2+y^2=12,x+y=4.求xy的值.
(x+y)^2=x^2+2xy+y^2
16=12+2xy
2xy=4
xy=2
2.99乘101乘10001
=(100-1)(100+1)10001
=(100^2-1)(100^2+1)
=100^4-1
=99999999