先化简,再给a、b各选一个你所喜爱的数代入求值.9ab-[(3a2+5ab-2b2)-(a2-3ab+b2)].
原式=9ab-[3a2+5ab-2b2-a2+3ab-b2]
=9ab-3a2-5ab+2b2+a2-3ab+b2
=ab-2a2+3b2
令a=1,b=1,
把a=1,b=1代入得:
1×1-2×12+3×12
=2.
先化简,再给a、b各选一个你所喜爱的数代入求值.9ab-[(3a2+5ab-2b2)-(a2-3ab+b2)].
原式=9ab-[3a2+5ab-2b2-a2+3ab-b2]
=9ab-3a2-5ab+2b2+a2-3ab+b2
=ab-2a2+3b2
令a=1,b=1,
把a=1,b=1代入得:
1×1-2×12+3×12
=2.