# How to do this question using completing the square.Watch

#1
The question is n^2 + 13n + 22 = 7

^ = power of...

¬ = square root

I got -6.5 = + or - ¬(-15). I'm guessing it's wrong though since you can't square root a negative number. So what's the real answer and your method of working it out would be appreciated. Thanks in advance!
0
7 years ago
#2
(Original post by Dr.Pwepper)
The question is n^2 + 13n + 22 = 7

^ = power of...

¬ = square root

I got -6.5 = + or - ¬(-15). I'm guessing it's wrong though since you can't square root a negative number. So what's the real answer and your method of working it out would be appreciated. Thanks in advance!
n2 + 13n + 22 = 7

n2 + 13n + 15 = 0

(n + 13/2)2 - (13/2)2 + 15 = 0

(n + 13/2)2 - 169/4 + 15 = 0

(n + 13/2)2 - 169/4 + 60/4 = 0

(n + 13/2)2 - 100/4 = 0

(n + 13/2)2 - 25 = 0

(n + 13/2)2 = 25

n + 13/2 = root 25

n + 13/2 = +/- 5

n = -6.5 +/- 5

n = -1.5 or -11.5

1
7 years ago
#3
-169/4 + 60/4 does not equal -100/4.

Replace with -109/4 which does not calculate as nicely.
1
7 years ago
#4

(Original post by thegodofgod)
(n + 13/2)2 - 169/4 + 60/4 = 0

(n + 13/2)2 - 100/4 = 0
Note sure I agree with your calculation in this step...
1
7 years ago
#5
(Original post by Chwirkytheappleboy)

Note sure I agree with your calculation in this step...
Woops - meant 109/4
0
7 years ago
#6
(Original post by thegodofgod)
Woops - meant 109/4
Easy mistake to make. It's generally a good idea to check your answer by plugging your solution into the equation to ensure it works.
0
7 years ago
#7
(Original post by Chwirkytheappleboy)
Easy mistake to make. It's generally a good idea to check your answer by plugging your solution into the equation to ensure it works.
I know - but the fact that the square root came out as an integer made me think that I was along the right track
0
#8
Thanks guys! I know what I did wrong now. I didn't add 42.25 to both sides of the equation. How do you close this thread haha
0
7 years ago
#9
It's against the rules to post answers, you're supposed to help users see where they've gone wrong .
0
7 years ago
#10
(Original post by ViralRiver)
It's against the rules to post answers, you're supposed to help users see where they've gone wrong .
I tend to agree, but I think in this case where it's such a simple question with such a straightforward answer, the best thing to do is just give the solution and then the person who asked the question can just see where they went wrong

(Original post by Dr.Pwepper)
Thanks guys! I know what I did wrong now. I didn't add 42.25 to both sides of the equation. How do you close this thread haha
Normal users can't close threads, we just let it die
0
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