①(-x-y)²
=[-(x+y)]²
=(x-y)²
=x²-2xy+y²
②201²
=(200+1)²
=200²+2×200×1+1²
=40000+400+1
=40401
③99.8²
=(100-0.2)²
=100²-2×100×0.2+0.2²
=10000-40+0.04
=9960.04