1.(2*3-1*6）x^2+(-5*2+3*3+1*7-1*6)xy-(3*5-1*7)y^2=-8y^2
2.（2a-b+c）（b-c-2a）=(2a-b+c)*-（2a-b+c)=-(2a-b+c）^2
3.99.8^2=(100-0.2)^2=100^2-2*100*0.2+0.2^2=10000-40+0.04=9960.04
4.11 101 （10^4+1）=10^2^2+1=100^2+1=10001
（10^8+1）=10^2^2^2+1=100^2^2+1=10000^2+1=100000001
5.6(x+y)^2-2(x-y)(x+y)=6(x+y)^2-2(x^2-y^2)
=6(x+y)^2-2x^2+^2y^2
=(6-2)x^2+(6*2)xy+(6+2)y^2
=4x^2+12xy+8y^2
=(2x+4y)(2x+2y)