Inverse proportionality

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BULL14

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n is proportional to to square root of m
n= 14 when m=49
find value of n when m =100

Please show working

davros

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(Original post by BULL14)
n is proportional to to square root of m
n= 14 when m=49
find value of n when m =100

Please show working

What have you tried so far?

If x is proportional to y, do you know how to convert this information into an equation relating x, y and some unknown constant k?

BULL14

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(Original post by davros)
What have you tried so far?

If x is proportional to y, do you know how to convert this information into an equation relating x, y and some unknown constant k?

14 is proportional to square root of m

14 = k/square root of 49
14*7 = k
k=98

davros

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(Original post by BULL14)
14 is proportional to square root of m

14 = k/square root of 49
14*7 = k
k=98

That looks like inverse proportion to me.

Should your question have said inverse proportion or just proportion?

BULL14

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inverse

davros

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(Original post by BULL14)
inverse

in that case I agree with your previous working

BULL14

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Well how would i find out if m=100

davros

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(Original post by BULL14)
Well how would i find out if m=100

You're supposed to be finding n,

You know what the value of k is now, so just put m = 100 into the formula.

BULL14

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n= 98/m
n=1.02

davros

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(Original post by BULL14)
n= 98/m
n=1.02

What happened to the square root?

And how did you get 1.02 as the answer?

BULL14

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so is it square root of 98 = 9.9 (to 1 d.p.)

davros

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(Original post by BULL14)
so is it square root of 98 = 9.9 (to 1 d.p.)

I don't understand what you're doing now - your first step should be to write down an equation relating m and n; you can then substitute values into this.

You also need to quote me if you want me to see that you've replied!

BULL14

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(Original post by davros)
I don't understand what you're doing now - your first step should be to write down an equation relating m and n; you can then substitute values into this.

You also need to quote me if you want me to see that you've replied!

can you just not tell me the answer?

joostan

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#14

(Original post by BULL14)
can you just not tell me the answer?

Yes, but what would you gain from that?

davros

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(Original post by BULL14)
can you just not tell me the answer?

Well, firstly TSR isn't a "do my homework" service, and secondly because I'm trying to help you learn so that you can do these problems independently. Otherwise you just end up with a list of answers to specific questions, not a general method.

Your posts have been a bit confused up to now - first you mentioned proportion, then square roots, then inverse proportion. So let's recap some rules:

if x is proportional to y, we can write:

where k is a constant
If x is inversely proportional to y, we can write $x = \dfrac{k}{y}$ where k is a constant
If x is proportional to the square root of y, we can write $x = k\sqrt{y}$ where k is a constant.

Am I correct in thinking that your question says "n is inversely proportional to the square root of m"? In which case, what do you think the rule should look like in terms of n, m and constant k?

You are given two values of n and m that work together. These should help you find k. Once you have k, you can just plug another value of m (e.g. m = 100) into the rule to find the n that corresponds to that value of m.

If you're still stuck, you might want to make your teacher aware that you're struggling a bit with this topic. Have you done some worked examples in class or from a text book?