2

# Calculus problem

Sat 15 Oct, 2011 12:26 pm
1.

2.

why
• Topic Stats
• Top Replies
Type: Question • Score: 2 • Views: 4,762 • Replies: 6
No top replies

markr

2
Sat 15 Oct, 2011 01:12 pm
@NapierE,
1. numerator is -infinity, denominator is 0 (essentially -infinity * infinity)
2. y^2 = 2x - x^2 + 9
differentiate both sides to get:

d/dy = 2y
d/dx = 2-2x

dy/dx = (2-2x )/ 2y
0 Replies

raprap

1
Sun 16 Oct, 2011 02:11 pm
@NapierE,
l'Hopital's rule

lim as x->0 f(x)/g(x)=lim as x>0 f'(x)/g'(x)

d[ln(x)]/dx=1/x and dx/dx=1

lim x->0 ln(x)/x=lim x->0 1/x=inf

Rap
markr

2
Sun 16 Oct, 2011 05:40 pm
@raprap,
L'Hopital's rule gives the wrong answer (wrong sign), and is not required. You don't need LH for (-infinity)/0.
0 Replies

witswang

1
Sun 17 Jun, 2012 07:40 pm
@raprap,
l'Hospital's rule requires the existence of f'(x) and g'(x) respectively, but when x tends to +0, the limit of lnx doesn't exist.
0 Replies

witswang

1
Sun 17 Jun, 2012 07:42 pm
@raprap,
l'Hospital's rule requires existence of f'(x) and g'(x) respectively. but, when x tends to +0, the limit of lnx doesn't exist, so the prerequisite to exert l'Hospital's rule doesn't meet, so it can't be used to solve the problem.
0 Replies

uvosky

1
Mon 6 Aug, 2012 05:55 am
''markr'' solved the problems in right way and ''raprap'' 's use of L'Hospital's law was wrong , but the reason why the law can not be used here is not exactly that presented by ''witswang'' , the L'Hospitals actually is;
when lim as x-->a f(x) & lim as x-->a g(x) are either both 0 or, both infinity
(or, - infinity)
then lim as x-->a f(x)/g(x) = lim as x-->0 f ' (x) / g ' (x) .
The reason why L'Hospital's law can not be used in the given problem is due to the fact that lim as x-->0+ lnx = - infinity but lim as x-->0+ x = 0
so both are not same hence condition of L'Hospital's law is not satisfied .
0 Replies

### Related Topics

STRAIGHT LINES - Question by iqrasarguru
Possible Proof of the ABC Conjecture - Discussion by oralloy
Help with a simple math problem? - Question by Anonymous1234567890
How do I do this on a ti 84 calculator? - Question by Anonymous1234567890
The bouncing ball - Question by whimsical
Alg. 1 Test prep Questions. - Question by Hay017
Quadratic formula quandry - Question by etrek

1. Forums
2. » Calculus problem