3a-2a^2+4-3a^2+a =（3+1）a+(-2-3)a²+4=4a-5a²+4xy+2xy-3x^2-3xy+x^2 =(1+2-3)xy+(-3+1)x²=-2x²5a^2b-3ab-4ab^2+7ab-2ab^2=5a²b+(-3+7)ab+(-4-2)ab²=5a²b+4ab-6ab² 4-2xy+7xy-15+xy =(4-15)+(-2+7+1)xy=-11+6xy
