解(1)
原式=-1/2×3x²-1/2×(-4xy)-1/2×(-5y²)+1/4×6x²+1/4×8xy-1/4×20y²
=-3/2x²+2xy+5/2y²+3/2x²+2xy-5y²
=(-3/2x²+3/2x²)+(2xy+2xy)+(5/2y²-5y²)
=0+4xy-5/2y²
=4xy-5/2y²
解(2)
原式=3x²-(2x²z-2xyz+x²z-4x²y)
=3x²-2x²z+2xyz-x²z+4x²y
=3x²+(-2x²z-x²z)+2xyz+4x²y
=3x²-3x²z+2xyz+4x²y
解(3)
原式=5xyz-[2x²y-(xyz+2x²y-xy)]
=5xyz-(2x²y-xyz-2x²y+xy)
=5xyz-2x²y+xyz+2x²y-xy
=(5xyz+xyz)+(-2x²y+2x²y)-xy
=6xyz-xy