1. anonymous

2. DanJS

put it in standard form first ax^2 + bx + c = 0 c is negative 5, not plus 5

3. anonymous

okay

4. anonymous

so the mistake starts from the first step?

5. Michele_Laino

hint: your equation is: ${x^2} + 4x - 5 = 0$

6. anonymous

i got that part:)

7. anonymous

a=1,b=4,c=-5

8. Michele_Laino

yes! the right answer is: $\begin{gathered} x = \frac{{ - 4 \pm \sqrt {{{\left( { - 4} \right)}^2} - 4 \times 1 \times \left( { - 5} \right)} }}{{2 \times 1}} = \hfill \\ \hfill \\ = \frac{{ - 4 \pm \sqrt {16 + 20} }}{2} = \frac{{ - 4 \pm 6}}{2} = ...? \hfill \\ \end{gathered}$

9. anonymous

1 and -5

10. Michele_Laino

that's right!

11. anonymous

but i need to explain where the student went wrong

12. anonymous

does she go wrong since the beginning because she put 5 instead of -5?

13. Michele_Laino

the student was wrong, since the coefficient c is -5 not +5

14. Michele_Laino

namely: $\Large c = - 5$ before applying the formula: $\Large x = \frac{{ - b \pm \sqrt {{b^2} - 4 \times a \times c} }}{{2 \times a}}$ we have to write our quadratic equation, in this form: $\Large a{x^2} + bx + c = 0$

15. anonymous

okay, thanks so much!! :)

16. Michele_Laino

thanks!