## lopus Group Title to melt a snowball, with initial radius of 12 cm, its radius decreases at a constant rate of 0.5 cm / hr. if it starts to melt when t = 0, the rate of decline in the volume of the snowball after 12 hours is: the answer is : A. 50pi cm3/hr B. 60 pi .. C. 72 pi .. D. 120pi .. E. 144pi .. one year ago one year ago

1. lopus

$v=\frac{ 4 }{ 3 }*R ^{3}$

2. satellite73

$V=\frac{4}{3}\pi r^3$ if i remember correctly taking derivatives you get $V'=4\pi r^2r'$

3. satellite73

you are told $$r'=-0.5$$ and you need $$V'$$

4. satellite73

oh and i guess after 12 hours $$r=6$$ so make the replacements and you are done

5. lopus

why 6? can you explain me

6. lopus

is 4pi(36)*0.5 = 72

7. lopus

why not 4pi(0.5*12)^2=144

8. satellite73

$$r=6$$ cm because it starts at 12 cm and decreases at a rate of 0.5 cm per hour

9. Skaematik

do you get this, OP?

10. lopus

is 4pi(36)*0.5 = 72 why not 4pi(0.5*12)^2=144

11. Skaematik

|dw:1353259541168:dw|

12. lopus

why not 4pi(0.5*12)^2=144