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in Factorization by (36.4k points)
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Factorize each of the following algebraic expressions:

x2 - 4x - 21

2 Answers

+1 vote
by (38.0k points)
selected by
 
Best answer

In order to factorize the given expression, we find to find two numbers p and q such that:

p + q = -4, pq = -21

Clearly, 3 – 7 = -4, 3 (-7) = -21

Therefore,

split (-4x) as 3x – 7x 

Therefore,

x2 + 4x – 21 = x2 + 3x – 7x – 21

= x (x + 3) – 7 (x + 3)

= (x – 7) (x + 3)

+1 vote
by (20 points)
x² - 4x - 21

x² - 7x + 3x - 21

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

(x-7)(x+3)

It's done!

To find zeroes,it is done like this:-

(x-7) (x+3) =0

(x-7)=0      → x=7

(x+3)=0     → x=-3

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