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2. Polynomials
hard
If both $x-2$ and $x-\frac{1}{2}$ are factors of $p x^{2}+5 x+r,$ show that $p=r$
Option A
Option B
Option C
Option D
Solution
Let $p(x)=p x^{2}+5 x+r$
As $(x-2)$ is a factor of $p(x)$
So, $p(2)=0 \Rightarrow P(2)^{2}+5(2)+r=0$
$\Rightarrow \quad 4 p+10+r=0…..(1)$
Again, $\left(x-\frac{1}{2}\right)$ is factor of $p(x)$
$\therefore \quad p\left(\frac{1}{2}\right)=0$
Now, $\quad p\left(\frac{1}{2}\right)=p\left(\frac{1}{2}\right)^{2}+5\left(\frac{1}{2}\right)+r$
$=\frac{1}{4} p+\frac{5}{2}+r$
$\therefore \quad p\left(\frac{1}{2}\right)=0 \Rightarrow \frac{1}{4} p+\frac{5}{2}+r=0….(2)$
From $(1),$ we have $4 p+r=-10$
From $(2),$ we have $p+10+4 r=0$
$\Rightarrow \quad p+4 r=-10$
$\therefore \quad 4 p+r=p+4 r$ $[\because$ Each $=-10]$
$\therefore \quad 3 p=3 r \Rightarrow p=r$
Hence, proved.
Standard 9
Mathematics