Re-arranging formulae

Monkeyseat

Full Member
Joined
Jul 3, 2005
Messages
298


(^ Make sure you can see pictures ^)

I do this:

Square both sides:

A^2=3P

Divide by 3:

(A^2)/3 = P

P = (A^2)/3

Yet the answers say it is meant to be:

P=3A^2



Where am I going wrong?

Cheers.
 

morson

Full Member
Joined
Apr 12, 2007
Messages
263
Your solution is \(\displaystyle P = \frac{A^2}{3}\\)

Let's substitute our solution into the equation to check:

\(\displaystyle A = \sqrt{3 \frac{A^2}{3}\ }\ = \sqrt{A^2}\\)

This is only true for non-negative A.

So, let's revise the solution:

\(\displaystyle P = \frac{A^2}{3}\\), for non-negative A.
 

Monkeyseat

Full Member
Joined
Jul 3, 2005
Messages
298
Sorry, so which solution was correct? Is the answer from the book incorrect?

Thanks.
 

morson

Full Member
Joined
Apr 12, 2007
Messages
263
The book's answer is not correct in the general case, no.
 

Monkeyseat

Full Member
Joined
Jul 3, 2005
Messages
298
Oh right, thanks.

It wasn't really a book, it was the mark scheme for a GCSE test so I didn't think it would be wrong but I couldn't find how they got that answer...

Thanks for the help.
 
Top