How do you solve x^2 + 2x - 15 = 0 ?

2 Answers
Jim G.
Apr 12, 2018

x=-5" or "x=3

Explanation:

"the factors of - 15 which sum to + 2 are + 5 and - 3"

rArr(x+5)(x-3)=0

"equate each factor to zero and solve for x"

x+5=0rArrx=-5

x-3=0rArrx=3

Watchmojo J.
Apr 12, 2018

x=-5 or 3

Explanation:

Rules for factorising:

  • ax^2+bx+c=0
  • m xx n = c
  • m + n = b
  • :.(x+m)(x+n) = 0

In this case:

5 xx -3 = -15 = c

5 + -3 = 2 = b

:. x^2+2x−15=0 -> color(red)((x+5)(x-3) = 0)

Either of the brackets must be equal to 0.

Assuming (x+5) = 0:

color(red)(x=-5)

Assuming (x-3) = 0:

color(red)(x=3)

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