Question

Como resolver equação 
2x(4x-1)=21

Answer 1

$2x(4x-1)=21$

$8x^2-2x=21$

$8x^2-2x-21=0$

$\Delta=(-2)^2-4\cdot8\cdot(-21)=4+672=676$

$x=\dfrac{-(-2)\pm\sqrt{676}}{2\cdot8}=\dfrac{2\pm26}{16}$

$x'=\dfrac{2+26}{16}=\dfrac{28}{16}=\dfrac{7}{4}$

$x"=\dfrac{2-26}{16}=\dfrac{-24}{16}=\dfrac{-3}{2}$