Question

[Dang Dao Chau Giang]

Tìm x :

a) (2x-1)³-4x² . (2x-3)=5

b) (x+1)³-(x-1)³-6.(x+1)²=-10

c) (x+2)³-x².(x+6)=4

Answer 1

[Nguyễn Đức Trí]

a) \(\left(2x-1\right)^3-4x^2\left(2x-3\right)=5\)

\(\Leftrightarrow8x^3-12x^2+6x-1-8x^3+12x^2=5\)

\(\Leftrightarrow6x-1=5\Leftrightarrow6x=6\Leftrightarrow x=1\)

b) \(\left(x+1\right)^3-\left(x-1\right)^3-6\left(x+1\right)^2=-10\)

\(\Leftrightarrow\left(x+1-x+1\right)\left[\left(x^2+2x+1+x^2-2x+1+\left(x^2-1\right)\right)\right]-6\left(x^2+2x+1\right)=-10\)

\(\Leftrightarrow2\left(3x^2+1\right)-6x^2-12x-6=-10\)

\(\Leftrightarrow6x^2+2-6x^2-12x-6=-10\)

\(\Leftrightarrow-12x-4=-10\Leftrightarrow12x=-6\Leftrightarrow x=\dfrac{1}{2}\)