Question

Tìm x, biết :

a) 5x . (x - 3)² - 5 . (x - 1)³ +15 . (x + 2) . (x - 2) = 5

b) (x + 3)³ - x . (3x + 1)² + (2x +1) . (4x² - 2x +1 ) - 3x² = 42

Answer 1

\(a,5x\left(x-3\right)^2-5\left(x-1\right)^3+15\left(x+2\right)\left(x-2\right)=5\)\(\Leftrightarrow5x\left(x^2-6x+9\right)-5\left(x^3-3x^2+3x-1\right)+15\left(x^2-4\right)-5=0\)\(\Leftrightarrow5x^3-30x^2+45x-5x^3+15x^2-15x+5+15x^2-60-5=0\)\(\Leftrightarrow30x=60\Leftrightarrow x=2\)

\(b,\left(x+3\right)^3-x\left(3x+1\right)^2+\left(2x+1\right)\left(4x^2-2x+1\right)-3x^2=42\)\(\Leftrightarrow x^3+9x^2+27x+27-x\left(9x^2+6x+1\right)+8x^3+1-3x^2-42=0\)\(\Leftrightarrow x^3-9x^2+27x+27-9x^3-6x^2-x+8x^3+1-3x^2-42=0\)\(\Leftrightarrow26x=14\Rightarrow x=\dfrac{7}{13}\)

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