Solve the following quadratic equation by factorisation method:
$x^{2} + x - 20 = 0$
$[2]$

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Solution

Solution:
$x^{2} + x - 20 = 0$
Split -20 into two such factors such that their sum equal to 1.
-20 = -4 $\times$ 5 and also, sum of -4 and 5 is 1.

$∴$ By factorisation method,
$\Rightarrow x^{2} + 5 x - 4 x + 20 = 0$ [1 Mark]
$\Rightarrow x (x + 5) - 4 (x + 5) = 0$
$\Rightarrow (x + 5) (x - 4) = 0$
$\Rightarrow x + 5 = 0 o r x - 4 = 0$
$\Rightarrow x = - 5 o r x = 4$ .
[1 Mark]

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