Don't even know where to start..
volume of a sphere has decreased by 30 percent. how many percentage has the radius decreased?

2. ## Re: volum and radius

When the radius of a sphere is multiplied by r,
the area is multiplied by r^2
the volume is multiplied by r^3.

So when the volume is multiplied by .7 = r^3,
r = ...
(Then subtract this from "1", as I did to get .7, and convert to a percentage if needed)

3. ## Re: volum and radius

aha.. i realy don't get it :/

4. ## Re: volum and radius

Originally Posted by tintin
aha.. i realy don't get it :/
Well, we could break this into two parts.
1) Concluding that r^3 = .7
2) Solving for r in the equation in (1).

Which don't you get?

5. ## Re: volum and radius

I don't know what you mean r^3 = .7.

6. ## Re: volum and radius

Tintin, did you at least google "volume of sphere"?
Is this a problem given to you by your math teacher?

7. ## Re: volum and radius

No its in my math book. I managed doing it the other way around, like ''the radius of a sphere grows by 12,5 percent, how many percentage will the volume increase?''.
Where i got ((1.125^3) -1)*100)=42%
I just don't get it when it decreases..

8. ## Re: volum and radius

HINT:
9^3 = 729 : 10 reduced by 10% to 9)
10^3 = 1000
11^3 = 1331 : 10 increased by 10% to 11)

9. ## Re: volum and radius

Hello tintin,
V sphere =4/3 pi R^3
V2 /V1 = .7/1= 4/3 pi R2^3/4/3pi R1^3
V2/V1 = .7= R2^3/R1^3
.7^1/3 = R2/R1 .887 R1 =R2