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Mrs Tibble’s Year 11209 comments to Mrs Tibble’s Year 11 |
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Hi Zoe,
For inverse proportion, the product is constant
eg if P is inversely proportional to V, then P x V = k (the constant of proportionality)
so every pair of P and V values will multiply to give you that same answer (k)
If there are powers, then remember to apply them:
eg A is inversely proportional to B-squared
then A x B-squared gives you the constant, not A x B
With this one, if you have to find B, then the proportionality will give you B-squared and you need to remember to square root it at the end.
Try here for some practice questions:
http://www.mathsmadeeasy.co.uk/GCSE-maths-revision/prop-inverse/direct-inverse-proportions.htm
Have fun
Mrs T
Thank you very much. Could u possibly type out a question and a solution? So i can see the step by step method?
Hi Zoe,
Q. The pressure of a gas P is inversely proportional to its volume V. V is 10 litres when the pressure is 4 atmospheres.
a) what is the pressure when the volume is 8 litres?
b) what is the volume when the pressure is 2.5 atmospheres?
Answer:
P is inversely proportional to V so PV=k
From info at start, k = 10 x 4 = 40 (note: the units don’t matter so long as they are always the same in each part of the question)
Now you know the constant is 40 so every pair of values must multiply to 40.
a) P x 8 = 40 therefore P = 40/8 = 5 litres
b) 2.5 x V = 40 therefore V = 40/2.5 = 16 atmospheres
Does that help?
Mrs T
Yes great, thank you!
Hi Zoe,
The difference with direct proportion is that you divide to get the constant.
Mrs T
Hello sorry for the question that you set us from the exam, I’m finding it really difficult to figure out (b) of the question. I found the side of CD using pythagorus and got the answer 11.1cm but now I’m beginning to doubt that because I can’t find the other side. How do I find the other side? Thank you
Hi Ella,
You need Pythagoras in the first triangle, which is what you did. (root 55)
Next you need trig (SOH CAH TOA) for the right angled triangle in the middle.
Lastly you need the cosine rule in the third triangle.
Answers are 9.68cm and 13.1cm
Hi Mrs Tibble I’m not sure what to do for exercise 20D question 5a?
Hi Sophie,
Sorry – only just seen this. It’s the Fibonacci sequence.
Mrs T