(5.2x²y-3.4xy+8x³)+2(2.2xy-4x³-2.3x²y)
=5.2x²y-3.4xy+8x³+4.4xy-8x³-4.6x²y
=0.6x²y+xy
=0.6×(-1/2)²×5-1/2×5
=3/4-5/2
=-7/4