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Precalc Help

Blogs > FragKrag
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FragKrag
Profile Blog Joined September 2007
United States11552 Posts
Last Edited: 2008-10-22 03:28:28
October 01 2008 04:07 GMT
#1
hihi T_T

New Problem:X^3+Y^3=125

dy/dx =?
a. -x/y
b.-x/5y
c. y/x
d. x/(y-5)
e. x/y

Well, I thought it was -(x/y)^2, but it's not on there.
I think that it should be negative, and there should not be a 5, which leaves me -x/y, which is x^2+y^2=C

Problem: The function y=|x-8|+2 is differentiable over..

a. [8,10)
b. (2,10)
c. [2,10]
d. [2,10)
e. (2,10]

halps would be appreciated <3

Edit: What we got is that since [8,10) has a continuous slope of 1, it's right.


*
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
Saracen
Profile Blog Joined December 2007
United States5139 Posts
October 01 2008 04:11 GMT
#2
graph it
if the sharp point from the absolute value is included in your set of points, then no
if not, then yes
doghunter
Profile Blog Joined July 2008
United States23 Posts
October 01 2008 04:13 GMT
#3
It's not differentiable at x=8 (cusp), so I thought the answer would be (-inf,8),(8,inf) but that doesn't seem to be an answer.
Avidkeystamper
Profile Blog Joined June 2008
United States8552 Posts
October 01 2008 04:13 GMT
#4
It's a, all the other intervals include the corner of the V the graph produces and that corner is not differentiable since the slope from both sides is not the same. Notice how a does not include 8 in its interval since that point is part of the corner and is not differentiable (<-I think?)
Jaedong
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
October 01 2008 04:18 GMT
#5
We did graph it, and it's a V, but 2,10 all contain two slopes, 1 and -1 as derivatives.

*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
doghunter
Profile Blog Joined July 2008
United States23 Posts
October 01 2008 04:32 GMT
#6
There's no limit from the left side, so you can't say that there is a limit at x=8
vAltyR
Profile Blog Joined July 2008
United States581 Posts
October 01 2008 04:36 GMT
#7
what it's asking is if you get a continuous derivative over the intervals. If I'm interpreting the equation right, the point of the "V" should be at x=8, therefore the derivative at x=8 does not exist, eliminating all the intervals between 2 and 10, whether or not they actually *include* 2 or 10. the answer is a.

man, that's a *precalc* question? we never covered that until calculus. in fact, i never even heard the word "derivative" until i opened a calc book.
내 호버크라프트는 장어로 가득 차 있어요
thunk
Profile Blog Joined March 2008
United States6233 Posts
Last Edited: 2008-10-01 04:50:28
October 01 2008 04:48 GMT
#8
8 is a point of non-differentiability, so I think that (a) is also wrong. But for sure b-e are wrong more so, so it's definitely A.

On October 01 2008 13:32 doghunter wrote:
There's no limit from the left side, so you can't say that there is a limit at x=8


It's continuous and the limit as x approaches 8 exists. It's just no differentiable because the derivative from the right and the derivative from the left are inequal.
Every time Jung Myung Hoon builds a vulture, two probes die. || My post count was a palindrome and I was never posting again.
Purind
Profile Blog Joined April 2004
Canada3562 Posts
October 01 2008 14:56 GMT
#9
See I thought a would be wrong too, but if you only graph that interval, you get a straight line and you don't see the pointiness
Trucy Wright is hot
SIUnit
Profile Joined December 2006
China288 Posts
October 01 2008 16:29 GMT
#10
Asia tend to be a bit crazy on the math side of things. And yeah, this would technically be a calc 1 question here in the states.
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
October 22 2008 03:28 GMT
#11
bump for new question~
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
skyglow1
Profile Blog Joined April 2005
New Zealand3962 Posts
Last Edited: 2008-10-22 15:40:28
October 22 2008 04:46 GMT
#12
Use implicit differentiation which involves chain rule.
x^3+y^3=125
Differentiation both sides by x: 3x^2+d(y^3)/dx=0
Since y is a function of x, and using chain rule: d(y^3)/dx = 3y^2 x dy/dx
So you get: 3x^2 + 3y^2 x dy/dx = 0
And rearranging: dy/dx = -3x^2/3y^2

[Edit] I'm a retard -_-
SaveYourSavior
Profile Blog Joined July 2008
United States1071 Posts
October 22 2008 08:18 GMT
#13
what i got was -x^2 / y^2

but

a seems to be the right answer since it seems the most right
a
Divinek
Profile Blog Joined November 2006
Canada4045 Posts
October 22 2008 08:30 GMT
#14
On October 22 2008 13:46 skyglow1 wrote:
Use implicit differentiation which involves chain rule.
x^3+y^3=125
Differentiation both sides by x: 3x^2+d(y^3)/dx=0
Since y is a function of x, and using chain rule: d(y^3)/dx = 3y^2 x dy/dx
So you get: 3x^2 + 3y^2 x dy/dx = 0
And rearranging: dy/dx = -3x^2/3y^2 = -x/y so the answer should be a)


Implicit differentiation is in pre calc?
Never attribute to malice that which can be adequately explained by stupidity.
Oh goodness me, FOX tv where do you get your sight? Can't you keep track, the puck is black. That's why the ice is white.
Ecael
Profile Joined February 2008
United States6703 Posts
Last Edited: 2008-10-22 09:11:32
October 22 2008 09:10 GMT
#15
On October 22 2008 17:30 Divinek wrote:
Show nested quote +
On October 22 2008 13:46 skyglow1 wrote:
Use implicit differentiation which involves chain rule.
x^3+y^3=125
Differentiation both sides by x: 3x^2+d(y^3)/dx=0
Since y is a function of x, and using chain rule: d(y^3)/dx = 3y^2 x dy/dx
So you get: 3x^2 + 3y^2 x dy/dx = 0
And rearranging: dy/dx = -3x^2/3y^2 = -x/y so the answer should be a)


Implicit differentiation is in pre calc?

If memory serves schools in Taiwan covered power rule to implicit differentiation in precalc, at least that's what I recall of what my friends/parents/family's talks of how they had it tough :p

EDIT - It'd be hilarious when OP posts to say how he isn't at Taiwan though.
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
Last Edited: 2008-10-22 14:59:35
October 22 2008 14:56 GMT
#16
I thought it was a nationality? T__________T

I'm taking "PreCalculus". It's the course name, but the book has PreCalc and Calc I so lol tt;;

anyways, how can you go from -(x/y)^2 to -x/y? ?:/

*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
skyglow1
Profile Blog Joined April 2005
New Zealand3962 Posts
October 22 2008 15:41 GMT
#17
Sorry I'm so retarded. I shouldn't try to help at math when I don't learn it anymore
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
Last Edited: 2008-10-23 00:50:08
October 23 2008 00:49 GMT
#18
;_________;

Today my math teacher tells me that "abcde" is a choice when you cant find the answer. --;
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
onihunter
Profile Blog Joined September 2008
United States515 Posts
October 23 2008 01:23 GMT
#19
On October 23 2008 09:49 FragKrag wrote:
;_________;

Today my math teacher tells me that "abcde" is a choice when you cant find the answer. --;


Nice, would explain a lot about why I couldn't figure this problem out lol. And man I hate it when math questions have "None of the above" as a choice.
jaedong forever~
Ecael
Profile Joined February 2008
United States6703 Posts
October 23 2008 02:40 GMT
#20
On October 23 2008 09:49 FragKrag wrote:
;_________;

Today my math teacher tells me that "abcde" is a choice when you cant find the answer. --;

Ah good, I was going a bit insane and trying to log this through to solve earlier.
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