Question

Solve the following quadratic equation by factorisation.

x² + x – 20 = 0

Answer 1

 x² + x – 20 = 0  

\[\Rightarrow x^2 + 5x - 4x - 20 = 0\]
\[ \Rightarrow x\left( x + 5 \right) - 4\left( x + 5 \right) = 0\]
\[ \Rightarrow \left( x - 4 \right)\left( x + 5 \right) = 0\]
\[ \Rightarrow \left( x - 4 \right) = 0 \text{ or } \left( x + 5 \right) = 0\]
\[ \Rightarrow x = 4 \text{ or} x = - 5\]  

So, 4 and \[-5\] are the roots of the given quadratic equation.