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Inverse proportion help? Year 9 work...

2a) Two variables, a and b, are known to be proportional to each other.
When a= 2, b= 4.

Find the constant of proportion, k if:

i) a∝b^2

My working:
a=kb^2
2=kx16

Is k 1/8?

ii) a∝1 over square root of b (sorry, don't know how to type it out)

My working:
2= k over square root of 4
k= 2x2

Is k 4?

This is the part I don't know how to do...

b) y is inversely proportional to the cube root of x.

When y= 10, x= 8.
Find the value of y when x= 125.

Thanks in advance.
Reply 1
Avatar for TenOfThem
TenOfThem
18
Original post by no_name123


This is the part I don't know how to do...

b) y is inversely proportional to the cube root of x.

When y= 10, x= 8.
Find the value of y when x= 125.

Thanks in advance.


If p and q are inversely proportional then pq=k

Yes to the first 2 parts
Reply 2
Avatar for no_name123
no_name123
OP
Original post by TenOfThem
If p and q are inversely proportional then pq=k

Yes to the first 2 parts


So y x 5 = k ?

I don't understand the cube root part of the question.
Original post by no_name123
2a) Two variables, a and b, are known to be proportional to each other.
When a= 2, b= 4.

Find the constant of proportion, k if:

i) a∝b^2

My working:
a=kb^2
2=kx16

Is k 1/8?

ii) a∝1 over square root of b (sorry, don't know how to type it out)

My working:
2= k over square root of 4
k= 2x2

Is k 4?

This is the part I don't know how to do...

b) y is inversely proportional to the cube root of x.

When y= 10, x= 8.
Find the value of y when x= 125.

Thanks in advance.


y=\dfrac{k}{\sqrt[3] x}

Can you rearrange for k?

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Reply 4
Avatar for no_name123
no_name123
OP
Original post by majmuh24
y=\dfrac{k}{\sqrt[3] x}


Can you rearrange for k?

Posted from TSR Mobile

k = y*symbol for cube root of x*?
Original post by no_name123
k = y*symbol for cube root of x*?


Yep, so now you have all your numbers, so just sub back in :smile: This is supposed to be the easy part :wink:

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Reply 6
Avatar for no_name123
no_name123
OP
Original post by majmuh24
Yep, so now you have all your numbers, so just sub back in :smile: This is supposed to be the easy part :wink:

Posted from TSR Mobile


Ahh, I get it now, thank you so much!!! :biggrin:



Original post by TenOfThem
If p and q are inversely proportional then pq=k

Yes to the first 2 parts


Thanks to you too :smile:
Original post by no_name123
Ahh, I get it now, thank you so much!!! :biggrin:

Thanks to you too :smile:


No problem :smile:

Posted from TSR Mobile

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