1.（x-3y)(x+3y)-(x-3y)^2
=(x-3y)[(x+3y)-(x-3y)]
=6y(x-3y)
2.98^2
=(100-2)^2
=100^2-2x100x2+2^2
=10000-400+4
=99604
3.(2x-y-3)(2x+y+3)
=(2x)^2-(y+3)^2
=4x^2-y^2-6y-9
4.50.1^2
=(50+0.1)^2
=50^2+2x50x0.1+0.1^2
=2500+10+0.01
=2510.01
5.(5x+7y)^2-(5x-7y)^2
=[(5x+7y)+(5x-7y)][(5x+7y)-(5x-7y)]
=140xy