Cách làm cho bạn:
a) P + (x2 – 2y2) = x2 – y2 + 3y2 – 1
=> P = (x2 – y2 + 3y2 – 1) – (x2 – 2y2)
=> P = x2 – y2 + 3y2 – 1 – x2 + 2y2
=> P = (x2 – x2) + (-y2 + 3y2 + 2y2) – 1
=> P = 4y2 – 1
b) Q – (5x2 – xyz) = xy + 2x2 – 3xyz + 5
=> Q = (xy + 2x2 – 3xyz + 5) + (5x2 – xyz)
=> Q = xy + 2x2 – 3xyz + 5 + 5x2 – xyz
=> Q = xy + (2x2 + 5x2) – (3xyz + xyz) + 5
=> Q = xy + 7x2 – 4xyz + 5
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