Make n the subject of the formula: $$ P=400n^2-1280 $$ Why is the answer $$ n= \sqrt\frac{P+1280}{400} $$ And not $$ n=\sqrt{\frac{P}{400}+1280} $$
Asked
Active
Viewed 47 times
2 Answers
2
It is entirely possible to divide by 400 first thing you do. However, when you do this, you have to divide every term of the equation by 400, including the negative 1280 term.
$$P = 400n^2 - 1280$$ $$\frac{P}{400} = n^2 - \frac{1280}{400}$$ $$\frac{P}{400} + \frac{1280}{400} = n^2$$ $$\frac{P + 1280}{400} = n^2$$ $$\sqrt{\frac{P + 1280}{400}} = n$$ $$n = \sqrt{\frac{P + 1280}{400}}$$
A simple example should make it quite easy to see why you have to do this.
$$2 + 2 = 4$$ $$2 + 1 = 2$$ $$3 = 2$$
So, in order to have all of mathematics work, this would be correct.
$$2 + 2 = 4$$ $$1 + 1 = 2$$ $$2 = 2$$
For more information look here: https://youtu.be/oOmxNr26fWs
Trashtalk
- 278
0
Hint: we have $$P+1280=400n^2$$ and then you must divide the whole sum by $$400$$ so you get $$\frac{P+1280}{400}=n^2$$ If you divide at first by $400$ you will get $$\frac{P}{400}=n^2-\frac{1280}{400}$$
Dr. Sonnhard Graubner
- 95,283
-
Why cant u divide first then plus. Is there any rule you must follow? – IZZLIEDAT Apr 21 '18 at 12:11
-
No, at first i have added $1280$ and then we can divide by $400$ – Dr. Sonnhard Graubner Apr 21 '18 at 12:15
-
Even if you divide everything by $400$ first regardless, you will still get the same answer. – Aurora Borealis Apr 21 '18 at 12:15
-
I dont think so. – IZZLIEDAT Apr 21 '18 at 12:20
-
by the second way you will get the same answer, why do you don't think so? – Dr. Sonnhard Graubner Apr 21 '18 at 12:21
-
Your answer and the possible answer are not equal. 1280 isnt 512000 – IZZLIEDAT Apr 21 '18 at 12:28
-
you would have to multiply 1280 by 400 so as to make them a single fraction – IZZLIEDAT Apr 21 '18 at 12:30
-
No you must divide $1280$ by $400$! – Dr. Sonnhard Graubner Apr 21 '18 at 12:33
-
you must times both numerator and denominator by 400 right? – IZZLIEDAT Apr 21 '18 at 12:36
-
400 is the common denominator – IZZLIEDAT Apr 21 '18 at 12:36