You are Here: Home >< Maths

# Inversely proportional question

1. (Original post by Fizzle-A)
So, whenever there is an 'inversely proportional' question, whatever happens to the one side, the opposite happens to the other?
Yes. The easiest way to see this, with equations which I would say are far easier to understand, is that if P = k (1/V), then multiplying by V, we get PV = k, or just PV = constant, which is the way that Boyle's law is usualy stated.

If you start off with pressure P1 and volume V1, and end up with pressure P2 and volume V2, then P1V1 = k and P2V2 = k. Since k is a constant, it is easier to write P1V1 = P2V2. From this, it is easy to see that P2 = (V1 / V2) P1. So if V2 = 1.25 V1, then V1 / V2 = V1 / 1.25V1 = 1 / 1.25, and you get P2 = P1 / 1.25 straight away - no need to write an equation with a k in at any point.

You can extend this to Charles' law (V1 / T1 = V2 / T2) and the pressure law (P1 / T1 = P2 / T2), and can even combine all three together to get P1V1 / T1 = P2V2 / T2. With these, you can solve all kinds of problems without having to worry about what the constants of proportionality actually are.

In every case, there are certain physical conditions that have to be satisfied, such as the temperature having to be constant for Boyle's law to hold.
2. (Original post by Fizzle-A)
So, whenever there is an 'inversely proportional' question, whatever happens to the one side, the opposite happens to the other?

V = k/P
1.25V = k/P/1.25
1.25V - k/1/1.25

1/1.25 = 0.8
A decrease of 20%
hey man can u please tell me how p becomes 1?
3. (Original post by Fizzle-A)
So, whenever there is an 'inversely proportional' question, whatever happens to the one side, the opposite happens to the other?

V = k/P
1.25V = k/P/1.25
1.25V - k/1/1.25

1/1.25 = 0.8
A decrease of 20%
pls can u help me with this?

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: April 6, 2017
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### A-level exams coming up?

Find everything you need here.

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams