Bài 4 :
a)$M=3,5x²y-2xy²+1,5x²y+2xy +3xy²$
=$(3,5x²y+1,5x²y)+(3xy²-2xy²)+2xy$
=$5x²y+xy²+2xy$
$N=2x²y+3,2xy+xy²-4xy²-1,2xy$
=$2x²y+(xy²-4xy²)+(3,2xy-1,2xy)$
=$2x²y -3xy²+2xy$
b) $M-N=(5x²y+xy²+2xy)-(2x²y -3xy²+2xy)$
=$5x²y+xy²+2xy-2x²y +3xy²-2xy$
=$(5x²y -2x²y)+(xy²+3xy²)+(2xy-2xy)$
=$3x²y + 4xy²$