GMATGuruNY
GMATGuruNY
MathRevolution
[Math Revolution GMAT math practice question]
When \(f(x)=ax^2+bx+c (a≠0)\), is \(x+1\) a factor of \(f(x)?\)
\(1) f(1)=0\)
\(2) f(-1)=0\)
(x + 1) is a factor of \(ax^2+bx+c\) if the quadratic is equal to 0 when x=-1.
[color=#ffff00]To illustrate:
\(x^2 + 4x + 3 = 0\)
\((x+1)(x+3) = 0\)
\(x=-1\) or \(x=-3\)
When \(x=-1\), \(x^2 + 4x + 3 = 1 - 4 + 3 = 0\).[/color]
[/highlight]
Question stem, rephrased:
Is the function equal to 0 when x=-1?
Hi
GMATGuruNY, Could you please explain how we get the highlighted part? I am not sure I follow the illustration.
To solve a quadratic, we generally set it equal to 0 and determine its factors, as follows:
\(x^2 - 2x = 1\)5
\(x^2 - 2x - 15 = 0\)
\((x-5)(x+3) = 0\)
x=5 or x=-3
The FACTORS of the quadratic are (x-5) and (x+3).
The ROOTS are x=5 and x=-3.
When a root is plugged into a factor, the factor has a value of 0.
If we plug x=5 into (x-5), we get:
5-5 = 0
Since a root causes one of the factors to have a value of 0, it also causes the quadratic itself to have a value of 0.
If we plug x=5 into \(x^2-2x-15\), we get:
\(5^2-2*5-15=0\)
If we know a factor of a quadratic, we can determine the value of one root -- a value that will cause both the factor and thus the quadratic itself to have a value of 0.
If (x-10) is a factor \(ax^2+bx+c\), then x=10 is a root, implying that x=10 will cause both (x-10) and \(ax^2+bx+c\) to have a value of 0.
If (x+7) is a factor \(ax^2+bx+c\), then x=-7 is a root, implying that x=-7 will cause both (x+7) and \(ax^2+bx+c\)to have a value of 0.
If (2x-3) is a factor \(ax^2+bx+c\), then x=3/2 is a root, implying that x=3/2 will cause both (2x-3) and \(ax^2+bx+c\) to have a value of 0.
The question stem above:
When \(f(x)=ax^2+bx+c (a≠0)\), is \(x+1\) a factor of \(f(x)?\)
For (x+1) to be a factor of \(ax^2+bx+c\), x=-1 must be a root, implying that x=-1 will cause both (x+1) and \(ax^2+bx+c\) to have a value of 0.
In other words:
If x=-1 causes the function to have a value of 0, then (x+1) will be a factor of the quadratic, and the answer to the question stem will be YES.
Rephrasing the question stem, we get:
Is the function equal to 0 when x=-1?Clear?[/quote]
Crystal Clear. Thank you sir!