If one root of the quadratic equation x^2 + ax - 3 = 0 is 3. What

Question

If one root of the quadratic equation  x^2 + ax - 3 = 0  is 3. What is the value of a?

A. -2
B. -1
C. 0
D. 1
E. 2

Sankriti · Answer

If one root is 3 then we can replace x with 3 in the equation
(3)^2+3a-3=0
9+3a-3=0
a=-2
Option A

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Soz · Answer

let the roots of a quadratic equation be r1 and r2. Here given that r1=3.

We know, r1+r2 = a and r1r2=-3.

r1=3
Hence for r1r2 to be -3, r2 must be -1. Hence a will be r1+r2 = 3+ -1 = 2.

What is wrong with this approach?

Fdambro294 · Answer

The sum of the roots of the quadratic equation are given by:

(-) (coefficient in front of X term)
_________________________
(Coefficient in front of X^2 term)

Or in this case:

(-) (a)
_____
(1)

r1 + r2 = (-)a

(r1) (r2) = -3

Let root 1 = r1 = 3

3 + r2 = (-)a

(3) (r2) = -3

r2 = root 2 = -1

Substituting:

3 + (-1) = (-)a

2 = (-)a

a = (-)2

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