So I am learning Quadratic Equations and I have learned about the Formulas for calculating the Delta $(\Delta)$ and $x_1$, $x_2$.

I have this equation $$x^2 - 10x + 15 = 0$$ and I've tried to do my best but it turns out that I have the wrong result on $x_2$ according to cymath.com.

Could someone explain what I've done wrong ?


$\Delta = b^2-4ac=100-4\times 15=100-60$

$\Delta = 40$

$\Delta > 0$


$x_1=\dfrac{-(-10)+\sqrt\Delta}{2}=\dfrac{10+\sqrt\Delta}{2} = 8.16$

$x_2=\dfrac{-(-10)-\sqrt\Delta}{2}=\dfrac{10-\sqrt\Delta}{2} = 3.67$

  • 1
    $\begingroup$ You seem to have only made an arithmetic mistake. $\frac{10 - \sqrt{40}}{2} \approx{} 1.84$. I think you forgot to divide by 2. $\endgroup$ – Mosquite Jun 19 '17 at 21:19

You didn't divide by $2$ $$10-\sqrt{40}\approx 3.67$$ Now you were asked for $$\frac{10-\sqrt{40}}{2}\approx\frac{3.67}{2}=1.835$$

  • $\begingroup$ :D HHHHH Thnx alot. I must have forgot to do that xD $\endgroup$ – Feelsbadman Jun 19 '17 at 20:35
  • $\begingroup$ @IdontReallywolf No problem,sometimes it happens to me too :P $\endgroup$ – kingW3 Jun 19 '17 at 20:36

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