Cách làm cho bạn:
a) P + Q = (x2y + x3 – xy2 + 3) + (x3 + xy2 – xy – 6)
= x2y + x3 – xy2 + 3 + x3 + xy2 – xy – 6
= x2y+ (x3+ x3)+ (– xy2+ xy2)+( 3– 6) - xy
= x2y + 2x3 -3 - xy
b) M + N = (x2y + 0,5xy3 – 7,5x3y2 + x3) + (3xy2 – x2y + 5,5x3y2)
= x2y + 0,5xy3 – 7,5x3y2 + x3 + 3xy3 – x2y + 5,5x3y2
= (x2y – x2y) + (0,5xy3+ 3xy3) + (– 7,5x3y2+ 5,5x3y2) + x3
= 3,5xy3 – 2x3y2 + x3
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