How do you factor the expression $x^{2} + 4 x - 21$ $x^2+4x-21$ ?

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How do you factor the expression $x^{2} + 4 x - 21$ $x^2+4x-21$ ?

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Answer 1 · 2018-03-06T17:21:16

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

Explanation:

To factorise any quadratic polynomial , you could use 2 methods.
Here's the easy one
Let the quadratic polynomial be $a x^{2} + b x + c$ $ax^2+bx+c$

▪multiply $a and c$ $a" and "c$ , then factorise the result into some factors, say y and z, $y + z = | b |$ $y+z=|b|$
$x^{2} + 4 x - 21$ $x^2+4x-21$ $(a \times c = - 21 = - 3 \times 7)$ $(a×c=-21=-3×7)$

$x^{2} + 7 x - 3 x - 21$ $x^2+7x-3x-21$

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

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

The other method is a direct application of quadratic formula.

Answer 2 · 2018-03-06T17:24:23

the answer is $(x + 7) (x - 3)$ $(x+7)(x-3)$

Explanation:

there's an easier way to find this but only when it's $x^{2}$ $x^2$ . since you know that two numbers have to multiply to give you $- 21$ $-21$ and add to give you $4$ $4$ when you find it, you don't have to go through the whole process and you can just put them in the parenthesis since there's no number in front of $x$ $x$

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How do you factor the expression $x^{2} + 4 x - 21$ $x^2+4x-21$ ?

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How do you factor the expression $x^{2} + 4 x - 21$ $x^2+4x-21$ ?