（5a-3a^2+1-4a^3)-(-2a^2-a^3)其中a=-2
=5a-3a²+1-4a³+2a²+a³
=5a-a²-3a³+1
=5*(-2)-(-2)²-3*(-2)³+1
=-10-4+24+1
=11
（9a²-12ab+5b²)-(7a²+12ab+7b²)其中a=1/2,b=-1/2
=9a²-12ab+5b²-7a²-12ab-7b²
=2a²-2b²-24ab
=2(a-b)(a+b)-24ab
=2(1/2+1/2)(1/2-1/2)-24*1/2*(-1/2)
=0+6
=6