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inversely proportional

need a bit of help with my gcse revision, have a mock tomorw :frown:

q is inversely proportional to the square of t
when t=4, q=5

(a) Find a formula for q in terms of t

I've totally forgotten how to do this!
Reply 1
Avatar for SimonM
SimonM
18
If two things are inversely proportional then they can be written as

a∝1b⇔a=kb\displaystyle a \propto \frac{1}{b} \Leftrightarrow a = \frac{k}{b}

In our case q∝1t2q \propto \frac{1}{t^2}
Reply 2
Avatar for Nandoz
Nandoz
OP
so if the value of t=5, how do you find q?

do you do q=1/25?
Reply 3
Avatar for JennyB
JennyB
No, you put in your initial known values into q = K/ t^2 to work out K

you then put t=5 into the formula q = k/125

And since you have just worked out K, you can then work out q
Reply 4
Avatar for SimonM
SimonM
18
Have you put t=4, q=5 into the formula?
Reply 5
Avatar for inkedup
inkedup
Hi, I,m new to all this and trying to get my head around something that i,m guessing is fairly simple :0. The question i have for my maths revision is as follows, and i,m trying to work out if i got it right.
If y∝ 1/x and x = 4 when y = 5, find the value of x when y = 10. Many thanks!!
y=kxy=\frac{k}{x} so yx=k yx = k

Does that help?
Reply 7
Avatar for steve2005
steve2005
17
Mr M
y=kxy=\frac{k}{x} so yx=k yx = k

Does that help?


The original post was a year ago. I expect the problem was solved many months ago!!!!
steve2005
The original post was a year ago. I expect the problem was solved many months ago!!!!


Use your eyes steve. I was replying to a post from a few minutes ago.
Reply 9
Avatar for inkedup
inkedup
Thanks for that, i kinda get that bit its working out the number bit. I,ve never done maths b4, so i expect i need a beginners guide!!
Reply 10
Avatar for marty.frmn
marty.frmn
q inversely prop. to t....
q=k/t^2
where k is a constant....
inkedup
Thanks for that, i kinda get that bit its working out the number bit. I,ve never done maths b4, so i expect i need a beginners guide!!


In your question y multiplied by x equals a constant k.

x = 4 when y = 5 so k = 20.

k will always equal 20 now.

If y = 10 what would x have to be?
Reply 12
Avatar for inkedup
inkedup
2!! Thanks very much, thats what i thought i just needed to know if i did it right!!
inkedup
2!! Thanks very much, thats what i thought i just needed to know if i did it right!!


The key fact to understand is that if two quantities are in direct proportion and one quantity doubles the other quantity also doubles.

If two quantities are in inverse proportion, if one doubles then the other one halves.

Note: this does not work where something is proportional or inversely proportional to the square or cube of something etc.

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