# Maths help!!

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#1
Hi I was wondering if anyone would be able to help me with a homework question:

P, Q and R have positive values.
P is directly proportional to the square of Q.
When P = 1.25, Q = 0.5
Q is inversely proportional to R.
When Q = 0.5, R = 6
Work out the value of R when P = 0.8

It’s probably really easy but idk where to start
Thanks in advance Last edited by 67824930; 1 month ago
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1 month ago
#2
Edit orry wrong post

Last edited by GabiAbi84; 1 month ago
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#3
(Original post by GabiAbi84)
Edit orry wrong post
Yeah sorry I edited it as I had managed to work out that one. Thanks though!
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1 month ago
#4
(Original post by 67824930)
Hi I was wondering if anyone would be able to help me with a homework question:

P, Q and R have positive values.
P is directly proportional to the square of Q.
When P = 1.25, Q = 0.5
Q is inversely proportional to R.
When Q = 0.5, R = 6
Work out the value of R when P = 0.8

It’s probably really easy but idk where to start
Thanks in advance So when know something is directly proportional how can we write that in an equation?
Last edited by GabiAbi84; 1 month ago
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#5
(Original post by GabiAbi84)
So when something is directly proportional how can we write that in an equation?
I think it’s y = kx but I don’t really understand what the k means
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1 month ago
#6
(Original post by 67824930)
I think it’s y = kx but I don’t really understand what the k means
K is a constant -a multiplier that shows the relationship between the two values.

So you know that P is directly proportional to the square of Q -how would you write that?
Last edited by GabiAbi84; 1 month ago
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#7
(Original post by GabiAbi84)
K is a constant -a multiplier that shows the relationship between the two values.

So you know that P is directly proportional to the square of Q -how would you write that?
P = kQ² ? Idk
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1 month ago
#8
(Original post by 67824930)
P = kQ² ? Idk
Yup that’s it.
Now use the values they give you
“When P=1.25, Q=0.5”
and put them into that equation to work out the value of k
Last edited by GabiAbi84; 1 month ago
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#9
(Original post by GabiAbi84)
Yup that’s it.
Now use the values they give you
“When P=1.25, Q=0.5”
and put them into that equation to work out the value of k
Okay so 1.25 = 0.25 x k
1.25 / 0.25 = 5
K = 5 ?
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1 month ago
#10
(Original post by 67824930)
Okay so 1.25 = 0.25 x k
1.25 / 0.25 = 5
K = 5 ?
👍🏼 now you can rewrite your formula for p and q using that value for k.

Now how do we write when something is inversely proportional?
Last edited by GabiAbi84; 1 month ago
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#11
(Original post by GabiAbi84)
👍🏼 now you can rewrite your formula for p and q using that value for k.

Now how do we write when something is inversely proportional?
y = k / x
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1 month ago
#12
(Original post by 67824930)
y = k / x
Excellent, now the question tells you that Q is inversely proportional to R.
Once you’ve written that equation, sub in the value the question gives you to find out k for this one.
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#13
(Original post by GabiAbi84)
Excellent, now the question tells you that Q is inversely proportional to R.
Once you’ve written that equation, sub in the value the question gives you to find out k for this one.
0.5 = k / 6
k = 3
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1 month ago
#14
(Original post by 67824930)
0.5 = k / 6
k = 3
Fab so you should now have two equations

One connecting P and Q and the other connecting Q and R
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#15
(Original post by GabiAbi84)
Fab so you should now have two equations

One connecting P and Q and the other connecting Q and R
I think this is wrong
P = 5Q²
Q = 3 / R
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1 month ago
#16
(Original post by 67824930)
I think this is wrong
P = 5Q²
Q = 3 / R
Why do you think it’s wrong?
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#17
(Original post by GabiAbi84)
Why do you think it’s wrong?
I’m not sure it was just a guess
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1 month ago
#18
(Original post by 67824930)
I’m not sure it was just a guess
Well it’s correct you worked out k and then you put those values back into the equations for proportional relations. Now you have those two equations, can you finish off the question?
Last edited by GabiAbi84; 1 month ago
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#19
(Original post by gabiabi84)
well it’s correct you worked out k and then you put those values back into the equations for proportional relations. now you have those two equations, can you finish off the question?
0.8 = 5q²
q = 0.4
0.4 = 3 / r
r = 7.5 ?
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1 month ago
#20
(Original post by 67824930)
0.8 = 5q²
q = 0.4
0.4 = 3 / r
r = 7.5 ?
That’s what I got Do you have an answer scheme to double check with?
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