化简：
898*902+4
=(900-2)*(900+2)+4
=900²-4+4
=900²
=810000
1)x²+2xy+y²-x-y
=(x+y)²-(x+y)
=(x+y)(x+y-1)
2)3x²y-6xy+3y²
=3y(x²-2x+1)
=3y(x-1)²