Formula: find the value.

jacob_1988

New member
Would it be be feasible for someone to help me fathom this 'quite simple' problem?

v = 1/3 πr2 h

Find the value of h if n = 3, r = 4 and v = 48. All I keep getting is 0.3 x 3 x 4 x 2 = 7.2

I have managed to answer another formula question like the one above, find s when u = 5, t = 2 and g= 10. s = ut + 1/2 gt2 (5 x 2 = 10. 0.5 x 10 x 2 x 2 = 20. 10 + 20 = 30. s = 30).

JeffM

Elite Member
Would it be be feasible for someone to help me fathom this 'quite simple' problem?

v = 1/3 πr2 h

Find the value of h if n = 3, r = 4 and v = 48. All I keep getting is 0.3 x 3 x 4 x 2 = 7.2

I have managed to answer another formula question like the one above, find s when u = 5, t = 2 and g= 10. s = ut + 1/2 gt2 (5 x 2 = 10. 0.5 x 10 x 2 x 2 = 20. 10 + 20 = 30. s = 30).
In your second problem, you are given a formula for the variable to be calculated

$$\displaystyle s = ut + \frac{1}{2}gt^2.$$

As you understand already, you simply insert the appropriate values in that case

$$\displaystyle s = 5 * 2 + \frac{1}{2} * 10 * 2^2 = 10 + 5 * 4 = 10 + 20 = 30.$$ Very good.

In your first problem, you are NOT given a formula for the variable to be calculated, but you are given a related formula. You must change the formula

$$\displaystyle v = \dfrac{1}{3} \pi r^2h$$ into a formula for h so that $$\displaystyle h = what?$$

How do you go about getting the formula you need from the formula you have? Please show us what you come up with and how you got there so we can tell you if you are correct and, if incorrect, why.

jacob_1988

New member
I'm probably making the same mistake but is the answer: 2 (3 x 4 = 12. 12 x 2 = 24. 24 x 2 = 48)? I think I'm wrong I will try again tomorrow

stapel

Super Moderator
Staff member
Would it be be feasible for someone to help me fathom this 'quite simple' problem?

v = 1/3 πr2 h

Find the value of h if n = 3, r = 4 and v = 48. All I keep getting is 0.3 x 3 x 4 x 2 = 7.2
What is the source of "0.3 x 3 x 4 x 2"? You are given a value of 3 for n and 4 for 4, but where is the "2" coming from? Where is "v = 48" used? Where did the 0.3 = 3/10 come from? What happened to the 1/3?

We can't trouble-shoot your work if you list only the final result.

I'm probably making the same mistake but is the answer: 2 (3 x 4 = 12. 12 x 2 = 24. 24 x 2 = 48)?
At a guess, you maybe are attempting guess-n-check (though I don't know the significance of the red-type formatting)...? So you are plugging "2" in for "h" and hoping it works...? But it doesn't:

. . . . .(1/3)(3)(4)^2 (h) = (1/3)(3/1)(16)(h) = 16(h) = 48

...and "h = 2" do not fit!

Have you studied exponents ("powers") at all? Because it seems like you maybe don't know what "r2" means...?

HallsofIvy

Elite Member
Would it be be feasible for someone to help me fathom this 'quite simple' problem?

v = 1/3 πr2 h

Find the value of h if n = 3, r = 4 and v = 48. All I keep getting is 0.3 x 3 x 4 x 2 = 7.2
One obvious difficulty is that "1/3" is NOT equal to ".3"! Do you know what (1/3)(3) is? And are you confusing "$$\displaystyle r^2$$", with "2r"? you should have $$\displaystyle 48= (1/3)(3)(4^2)h$$ and solve for h.

I have managed to answer another formula question like the one above, find s when u = 5, t = 2 and g= 10. s = ut + 1/2 gt2 (5 x 2 = 10. 0.5 x 10 x 2 x 2 = 20. 10 + 20 = 30. s = 30).

jacob_1988

New member
One obvious difficulty is that "1/3" is NOT equal to ".3"! Do you know what (1/3)(3) is? And are you confusing "$$\displaystyle r^2$$", with "2r"? you should have $$\displaystyle 48= (1/3)(3)(4^2)h$$ and solve for h.
Here it goes... (1.3)(3)(16) = 16. (16)(3) = 48? So h = 3?

1/3 x 3 = 1. 1 x 16 = 16. 16 x 3 = 48. h = 3?

jacob_1988

New member
SLOW DOWN, Jacob; you're running around like Skinny Minnie !

Your equation: v = 1/3 πr2 h where v=48, n=3, r=4

SAME as v = n(r^2)h / 3
So n(r^2)h = 3v

Then, h = ?

If you're unable to solve for h, then you're NOT ready for this.

NOTE to lookagain: I know the original is pi, not n
My last try...

h = 3V/πr²? h = 3v over
πr²?

p.s. does anyone know of any books(amazon.co.uk) or online work sheets whereupon I could practice some more formulas/volumes.

Last edited:

JeffM

Elite Member
My last try...

h = 3V/πr²? h = 3v over
πr²?

p.s. does anyone know of any books(amazon.co.uk) or online work sheets whereupon I could practice some more formulas/volumes.
$$\displaystyle v = \frac{1}{3} nr^2h \implies 3v = nr^2h \implies h = \dfrac{3v}{nr^2}.$$

But I doubt that formula is correct.

$$\displaystyle v = \frac{1}{3} \pi r^2h \implies 3v = \pi r^2h \implies h = \dfrac{3v}{\pi r^2}.$$ See whether that is the actual formula given.

To manipulate formulas of any type, you need to study basic algebra. If you do so, you will be able to work with formulas for many more things than just volumes.

JeffM

Elite Member
Jeff, we're using n because pi=3 is a given...and easier to type n=3
$$\displaystyle \pi = 3$$.

I see. Why in goodness name does the author of this problem expect people to be able to manipulate algebraic expressions but be unable to deal with a somewhat more exact value of pi, say 3.14 for example? Don't bother to answer; I'm sure any answer would drive me to strong drink.

pka

Elite Member
$$\displaystyle \pi = 3$$.
I see. Why in goodness name does the author of this problem expect people to be able to manipulate algebraic expressions but be unable to deal with a somewhat more exact value of pi, say 3.14 for example? Don't bother to answer; I'm sure any answer would drive me to strong drink.
Have a strong drink on me.
The Bible says that $$\displaystyle \pi=3$$. See 1 Kings 7:23.
That give folks who read the Bible literally fits.

JeffM

Elite Member
What? The drink or that tub?
The drink fit just fine. After a while I never even noticed that the tub was skewed. Oh, and thanks for the drink.