Let equation \(x^2+ax+b=0\) and \(x^2+cx+d=0 \) (a≠c) have the same root \(x\). Please determine this root value using a, b, c, d to represent it.

yasbib555 Sep 24, 2018

#1**+4 **

we can add the two equations to obtain

\(2x^2 +(a+c)x+(b+d)=0\\ x^2 + \dfrac{a+c}{2}x+\dfrac{b+d}{2}=0\)

\(\text{Now just apply the quadratic formula}\\ r_{1,2} = \dfrac{-\frac{a+c}{2}\pm \sqrt{\left(\frac{a+c}{2}\right)^2 - 2(b+d)}}{2}\)

\(\text{we can clean this up a bit}\\ r_{1,2} = \frac{1}{4} \left(\pm\sqrt{(a+c)^2-8 (b+d)}-a-c\right)\)

\(\text{If there is only a single shared root }r_1 = r_2\\ \text{if there are no shared roots}\\ (a+c)^2 - 8(b+d)<0\)

.Rom Sep 25, 2018

#2**+2 **

Why add the two equations ?

Why not simply subtract one from the other ?

Suppose that the first equation has roots p and q and that the second equation has roots p and s,

so that the common root is p.

Then we have

\(\displaystyle (x-p)(x-q)=0 \text{ and }(x-p)(x-s)=0\; .\)

Subtracting the second equation from the first,

\(\displaystyle (x-p)\{(x-q)-(x-s)\}=0\; ,\)

so

\(\displaystyle (x-p)(s-q)=0\; ,\)

showing that x = p is the root of the resulting equation, \(\displaystyle (\;s \neq q\;)\;.\)

So, going back to the original equations and subtracting one from the other,

\(\displaystyle x(a-c)+(b-d)=0\;,\)

\(\displaystyle x=\frac{d-b}{a-c}\; .\)

Tiggsy

Guest Sep 25, 2018