Đáp án+Giải thích các bước giải:

`a)` `3x(5x^2 - 2x - 1)`

`= 3x * 5x^2 - 3x * 2x - 3x * 1`

`= 15x^3 - 6x^2 - 3x`

`c)` `1/2 x^2y(2x^3 - 2/5 xy^2 - 1)`

`= (1/2 x^2y) * (2x^3) - (1/2 x^2y) * (2/5 xy^2) - (1/2 x^2y) * 1`

`= (1/2 * 2)x^(2+3)y - (1/2 * 2/5)x^(2+1)y^(1+2) - 1/2 x^2y`

`= x^5y - 1/5 x^3y^3 - 1/2 x^2y`

`e)` `1/2 xy(2/3 x^2 - 3/4 xy + 4/5 y^2)`

`= (1/2 xy) * (2/3 x^2) - (1/2 xy) * (3/4 xy) + (1/2 xy) * (4/5 y^2)`

`= (1/2 * 2/3)x^(1+2)y - (1/2 * 3/4)x^(1+1)y^(1+1) + (1/2 * 4/5)xy^(1+2)`

`= 1/3 x^3y - 3/8 x^2y^2 + 2/5 xy^3`

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