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

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

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

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

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