Question

Tìm x biết:
a)\(\left(x+3\right)^3-x\left(3x+1\right)^2+\left(2x+3\right)\left(4x^2-6x+9\right)-3x^2=25\)
b) \(\left(x-3\right)^3-\left(x-5\right)\left(x^2+5x+25\right)+6\left(x+2\right)^2+3x^2=11\)

Answer 1

a, \(\left(x+3\right)^3-x\left(3x+1\right)^2+\left(2x+3\right)\left(4x^2-6x+9\right)-3x^2=25\\ \Leftrightarrow x^3+27x^2+27-x\left(9x^2+6x+2\right)+8x^3+27-3x^2=25\\ \Leftrightarrow x^3+27x^2+27-9x^3-6x^2-2x+8x^3+27-3x^2=25\\ \Leftrightarrow18x^2-2x+54=25\\ \Leftrightarrow2x\left(9x-1\right)=-29\\ \Leftrightarrow\left\{{}\begin{matrix}2x=-29\\9x-1=-29\end{matrix}\right.\\ \Leftrightarrow x=-\dfrac{29}{2};x=-\dfrac{28}{9}\)