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\(a,=-15x^3+10x^4+20x^2\\ b,=2x^3+2x^2+4x-x^2-x-2=2x^3+x^2+3x-2\)

\(a,2x\left(3x^2+1\right)=6x^3+2x\)

\(b,\left(2x^3-5x^2+6x\right):2=x^3-\dfrac{5}{2}x^2+3x\)

\(c,\left(x-3\right)\left(x+5\right)-x\left(x+2\right)=x^2+5x-3x-15-x^2-2x=-15\)

\(\left(5x^2-2x+1\right)\left(2x^2-3x\right)=10x^4-15x^3-4x^3+6x^2+2x^2-3x\)

\(=10x^4-19x^3+8x^2-3x\)

Câu 3: 

a: 2x-8=4

nên 2x=12

hay x=6

b: 7x-3x=2x+7

\(\Leftrightarrow4x-2x=7\)

hay \(x=\dfrac{7}{2}\)

Câu 1:

a: \(5x\left(3x-4\right)=15x^2-20x\)

b: \(\left(x+5\right)\left(x-5\right)=x^2-25\)

c) \(5x^2.\left(3x^2-7x+2\right)\)

\(=5x^2.3x^2-5x^2.7x+5x^2.2\)

\(=15x^4-35x^3+10x^2\)

d) \(\dfrac{3x^2y^2+6x^2y^3-12xy}{3xy}\)

\(=\dfrac{3xy\left(xy+2y-4\right)}{3xy}\)

\(=xy+2y-4\)

\(a,=12x^2+18x-12x^2+x+1=19x+1\)

\(a,=12x^2+18x-12x^2+x+1=19x+1\\ b,=4x^3y^4-3xy^2-\dfrac{3}{2}x\)

a) \(\left(5x^2-2x+1\right)\left(2x^2-3x\right)\)

\(=10x^4-15x^3-4x^3+6x^2+2x^2-3x\)

\(=10x^4-19x^3+8x^2-3x\)

b) \(\left(18x^4y^3-6x^2y^3+12x^3y^4z\right)\)

\(=6x^2y^3\left(3x^2-1+2xyz\right)\)

\(\Rightarrow\left(18x^4y^3-6x^2y^3+12x^3y^4z\right):6x^2y^3\)

\(=\left[6x^2y^3\left(3x^2-1+2xyz\right)\right]:6x^2y^3\)

\(=3x^2+2xyz-1\)

c) \(\left(6x^3-7x^2-x+1\right)\)

\(=3x^2\left(2x+1\right)-5x\left(2x+1\right)+2\left(2x+1\right)-1\)

\(=\left(2x+1\right)\left(3x^2-5x+2\right)-1\)

\(\Rightarrow\left(6x^3-7x^2-x+1\right):\left(2x+1\right)\)

\(=\left[\left(2x+1\right)\left(3x^2-5x+1\right)-1\right]:\left(2x+1\right)\)

\(=3x^2-5x+1\) (Dư \(-1\))

a)(5x²-2x+1).(2x²-3x)

=10x⁴-4x³+2x²-15x³+6x²-3x

=10x⁴-19x³+8x²-3x

b)(18x⁴y³-6x²y³+12x³y⁴z):6x²y³

=(18x⁴y³:6x²y³)-(6x²y³:6x²y³)+(12x³y⁴z:6x²y³)

=3x²y-xy+2xyz