999^2-1002*998
=（1000-1)^2-(1000+2)(1000-2)
=1000^2-2000+1^2-(1000^2-2^2)
=1000000-2000+1-1000000+4
=5-2000
=-1995
(3)-20x^3+60x^2y-45xy^2
=-5x(4x^2-12xy+9y^2)
=-5X(2X-3Y)^2
(4)(x^2+2x)(x^2+2x-11)+24
=(x^2+2x)^2-11(x^2+2x)-24
=(x^2+2x-3)(x^2+2x-8)
先化简再求值:（5x-3)(5x+3)+(3x-2)(4-6x)-(3x-1)(3x+2),其中x=-2
25X^2-9-2(2-3X)(2-3X)-(3X-1)(3X-2)
=25X^2-9-2(4-6X+9X^2)-(9X^2-3X+2)
=25X^2-9-8+12X-18X^2-9X^+3X-2
=-2X^2+15X-19
=-2*4-15*2-19
=-57