squares

mslex1234

New member
Joined
Nov 9, 2010
Messages
2
I have a test tomorrow and we ran out of time in class today...the problem I am having is...

If 5 is added to the square of a number, the result is 21. Find the number.

x(x+5)=21

I think the answer is 4,-4 but do not really know how to work the problem out! Please help. And thank you in advanced. I am a bit frustrated...

The way I did it is...21-5 is 16, the square of 16 is 4. which makes it 4, -4
 

BigGlenntheHeavy

Senior Member
Joined
Mar 8, 2009
Messages
1,577
\(\displaystyle x^2 \ + \ 5 \ = \ 21.\)

\(\displaystyle x^2 \ = \ 21-5 \ =16.\)

\(\displaystyle \sqrt{x^2} \ = \ \sqrt{16}.\)

\(\displaystyle |x| \ = \ 4, \ \implies \ x \ = \ 4 \ or \ x \ = \ -4.\)

\(\displaystyle Note: \ By \ definition \ \sqrt{16} \ = \ 4, \ not \ -4.\)
 

mslex1234

New member
Joined
Nov 9, 2010
Messages
2
thank you so much for the quick response! :D
 
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