$x^{2} + 4 x - 21 = ? (a) (x - 7) (x + 3) (b) (x + 7) (x - 3) (c) (x - 7) (x - 3) (d) (x + 7) (x + 3)$

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Solution

$x^{2} + 4 x - 21 = x^{2} + 7 x - 3 x - 21 {\begin{matrix} ∵ - 21 = + 7 \times (- 3) 4 = 7 - 3 \end{matrix}} = x (x + 3) - 3 (x + 7) = (x + 7) (x - 3)$

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Similar questions

$(x^{2} - 4 x - 21)$ =?

(a) $(x - 7) (x - 3)$

(b) $(x + 7) (x - 3)$

(d) None of these

Factorise $2 x^{2} - 7 x + 6$

Tick $(✓)$ the correct answer in each of the following:
$(2 x + 3) (3 x - 1) = ?$

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(b) $(6 x^{2} + 7 x - 3)$
(c) $6 x^{2} - 7 x - 3$
(d) $(6 x^{2} - 7 x + 3)$

Factorise $x^{2} - 5 x + 6$ by using the Factor Theorem.

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