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6. Calculează: a) (2³-21²-1): 1; b) (12x+18r - 6x³): (62³); c) (42y³ - 2x²y - 4xy): (2xy); d) (3ry³ - 6xy² - 3y²): (3y²); e) (14r³y² - 7r³y): (7x³y); f) (2xyz +4r²z - 6xz²): (2xz).​