先化简,再求值.(3xy-7x²)-(-15x²+4xy),其中x=-1/2,y=-2

问题描述:

先化简,再求值.(3xy-7x²)-(-15x²+4xy),其中x=-1/2,y=-2

(3xy-7x²)-(-15x²+4xy),
=3xy-7x²+15x²-4xy
=-xy+8x²
=-(-1/2)*(-2)+8*1/4
=-1+2
=1