Question

Tìm x, biết:

a) 4x(5x+2)-(10x-3)(2x+7)=133

b) 4(x-1)(x+5)+(x+2)(x+5)=5(x-1)(x+2)

Answer 1

a) \(4x\left(5x+2\right)-\left(10x-3\right)\left(2x+7\right)=133\)

\(=\left(20x^2+8x\right)-\left(20x^2+64x-21\right)=133\)

\(=20x^2+8x-20x^2-64x+21=133\)

\(=-56x=112\)

\(\Leftrightarrow x=-2\)

b) \(4\left(x-1\right)\left(x+5\right)+\left(x+2\right)\left(x+5\right)=5\left(x-1\right)\left(x+2\right)\)

\(\Leftrightarrow\left(x+5\right)\left(4x-4+x+2\right)=5\left(x-1\right)\left(x+2\right)\)

\(\Leftrightarrow\left(x+5\right)\left(5x-2\right)=\left(5x-5\right)\left(x+2\right)\)

\(\Leftrightarrow5x^2+23x-10=5x^2+5x-10\)

\(\Leftrightarrow23x=5x\)

\(\Leftrightarrow18x=0\Leftrightarrow x=0\)

Answer 2

a) \(4x\left(5x+2\right)-\left(10x-3\right)\left(2x+7\right)=133\)

\(\Leftrightarrow20x^2+8x-\left[10x.\left(2x+7\right)-3.\left(2x+7\right)\right]=133\)

\(\Leftrightarrow20x^2+8x-\left(20x^2+70x-6x-21\right)=133\)

\(\Leftrightarrow20x^2+8x-20x^2-70x+6x+21=133\)

\(\Leftrightarrow8x-70x+6x=133-21\)

\(\Leftrightarrow-56x=112\)

\(\Leftrightarrow x=-\frac{112}{56}=-2\)

Vậy : \(x=-2\)