## bii17 3 years ago The base diameter and altitude of a right circulAR cone are observed at a certain instant to be 10 and 20 inches respectively. if the lateral area is constant and the base diameter is increasing at a rate of 1 inch per cubic minute, find the rate at ehich the altitude is decreasing

1. bii17

hmm yes.. he formula is LA = pi*r*L

2. Gowthaman

1 inch per cubic minute .... means what ? i am not understanding ' CUBIC MINUTE'

3. bii17

im sory it's 1 inch per minute

4. Gowthaman

LA = pi*r*L , and here L is slant height

5. bii17

yes...

6. Gowthaman

|dw:1348992383404:dw|

7. bii17

what happn nxt?

8. Gowthaman

is it 1/ sqrt(2) inch/ min

9. bii17

the answer will be 2.25 in per min @Gowthaman

10. Gowthaman

then somewhere i missed, i check once again

11. bii17

okay

12. Gowthaman

but now i got -.25 inch/min

13. Gowthaman

dh/dr =-1/2 (from condition of constant LS area) and dr/dt = 1/2 inch/min (given in the problem) dh/dt = dh/dr x dr/dt = - 1/2 x 1/2 = - 1/4 = -0.25 inch / min

14. Gowthaman

@bii17 ....?

15. sauravshakya

|dw:1348995046484:dw||dw:1348995119250:dw||dw:1348995143495:dw|

16. sauravshakya

|dw:1348995200114:dw|

17. sauravshakya

I hope u can solve it from here...

18. bii17

@sauravshakya hey stil there?

19. bii17

|dw:1349001368987:dw|

20. bii17

@sauravshakya what is that?? i didnt understand those with circle..

21. sauravshakya

That is radius of the cone

22. sauravshakya

Got it?

23. sauravshakya

ALSO,|dw:1349001642850:dw|

24. bii17

@hartnn can u help me in this question?

25. hartnn

which part of saurav's explanation did u not understand?

26. bii17

here.. |dw:1349002226151:dw| i used it but i didnt get the correct answer the answer should be 2.25 @hartnn

27. hartnn

@bii17 i did get it as 2.25. l^2=425, r=5, h=20, dr/dt=2

28. bii17

|dw:1349002789725:dw| i get is 8 @hartnn

29. hartnn

oh sorry, dr/dt = 1/2 and not 2

30. bii17

|dw:1349002976588:dw| my answer is 2.. :( @hartnn can u show me what u did?

31. hartnn

|dw:1349002984911:dw|

32. hartnn

did u get that u missed the -*minus* sign in the beginning ?