Question

When the square of a number is decreased by 15, it is equal to twice the original number. Find the possible values of the number.

• A. 8, 7
• B. 9, 6
• C. 2, -7
• D. 5, -3

Answer 1

The correct option is D

5, -3

Let the number be $x$.

Given, $x^2-15=2x$

$\Rightarrow$ $x^2-2x-15=0$

Factorising the quadratic equation, we get

$x^2-5x+3x-15=0$

$\Rightarrow$ $x(x-5)+3(x-5)=0$

$\Rightarrow (x-5)(x+3)=0$

$\Rightarrow$ $x=5orx=-3$

$\therefore$ The number can be 5 or -3.