Maths

If y is directly proportional to x^3 it means that y and x^3 increase at the same rate. So if you double the value for y, the corresponding value for x^3 is doubled. This means that y = k* x^3, where k is any constant.

So now you have a general equation of y = k * x^3, and you have a pair of values for x and y. Can you find the value for k? Try find the value for k to form a specific equation relating x to y.

(edited 4 years ago)

Reply 2

KlpDonnelly

OP

9

Original post by Maximus 190

If y is directly proportional to x^3 it means that y and x^3 increase at the same rate. So if you double the value for y, the corresponding value for x^3 is doubled. This means that y = k* x^3, where k is any constant.

So now you have a general equation of y = k * x^3, and you have a pair of values for x and y. Can you find the value for k? Try find the value for k to form a specific equation relating x to y.

That’s just confused me more?? Where did k come from.

Reply 3

Original post by KlpDonnelly

y is directly proportional to the cube of x.
When x =2, y = 56
Find the value of y when x=5

y=kx^3

56=k(2)^3
56=8k

divide both sides by 8 to get k on its own
k=7

plug into formula

y=7(5)^3
y=7(125)
y=875

Reply 4

Original post by hudz1e

y=kx^3

[redacted]

On TSR you aren’t meant to straight up post solutions, it doesn’t help anyone.

(edited 4 years ago)

Reply 5

Original post by KlpDonnelly

That’s just confused me more?? Where did k come from.

It’s just the general formula for directly proportional. If two things are directly proportional, for example if a and b are directly proportional, then a=kb. The k comes as it’s the gradient of b.

So if y= k*x^3, k is the gradient of x^3. If you drew a line, then k would be how steep the line is on the graph

Reply 6

Muttley79

20

Original post by hudz1e

y=kx^3

DO NOT post solutions - it's against the rules :smile:

Reply 7

Original post by Maximus 190

On TSR you aren’t meant to straight up post solutions, it doesn’t help anyone.

relax it's just one question. op can do more questions now after learning all the steps

Reply 8

omg whys everyone popping a vein.. if a teacher doesn't show u an example in school how are u ever gonna learn how to do it?

Reply 9

Original post by hudz1e

relax it's just one question. op can do more questions now after learning all the steps

Oh I was freaking out that you gave this guy the solution so thanks for telling me to relax, I’m now going to take 30 minutes out to meditate in my head space.

Reply 10

KlpDonnelly

OP

9

Original post by Maximus 190

On TSR you aren’t meant to straight up post solutions, it doesn’t help anyone.

None of it is helping to be honest I don’t understand any of it

Reply 11

Original post by KlpDonnelly

None of it is helping to be honest I don’t understand any of it

i'd recommend watching a video on mathsgenie until you get it

Reply 12