View Full Version : Differentiation problem
the flea
02-27-2012, 12:03 PM
I'm having serious problems solving this. Any help would be greatly appreciated.
http://i44.tinypic.com/n85ug.jpg
okokokok
02-27-2012, 12:09 PM
a:
What u will have to do is find the length of the ladders in the small triangles. So try to get the length of the ladder on the left from ladder b and the length of the ladder on the right of ladder a. If u have these lengths u can "flip" them over and have a bigger triangle to solve the length of the ladder AB.
the flea
02-27-2012, 12:10 PM
a:
What u will have to do is find the length of the ladders in the small triangles. So try to get the length of the ladder on the left from ladder b and the length of the ladder on the right of ladder a. If u have these lengths u can "flip" them over and have a bigger triangle to solve the length of the ladder AB.
how would this be done whilst only having one value of length and not knowing the angle x?
masterBB
02-27-2012, 12:11 PM
AB = b / cos(alpha) + a / sin(alpha)
the flea
02-27-2012, 12:12 PM
AB = b / cos(alpha) + a / sin(alpha)
I got that far earlier but hit a brick wall when I couldnt work out what the numerical value for alpha is
E: the differential of that equation is:
(67*sin(alpha)/cos(alpha)^2)-(13*cos(alpha)/sin(alpha)^2)
thats just even more confusing
okokokok
02-27-2012, 12:14 PM
I got that far earlier but hit a brick wall when I couldnt work out what the numerical value for alpha is
Let me take a look at it, it's not so easy for me since english is not my first language :) I'll get back to it in a few min
the flea
02-27-2012, 12:18 PM
Let me take a look at it, it's not so easy for me since english is not my first language :) I'll get back to it in a few min
thanks for your help.
This is what I have so far which is clearly wrong....
http://i44.tinypic.com/fubazl.jpg
okokokok
02-27-2012, 12:33 PM
I'll have to refresh my math a bit :P It's been a while since i have had this kind of math :P
okokokok
02-27-2012, 12:56 PM
I didn't really find a solution, but maybe i can help you get some ideas.
http://i39.tinypic.com/23to0ms.jpg
So what i was thinking is if:
ladder DL = tan (z) * 13 ( tan (z) = DL/13)
ladder CL = tan (x) * 67 ( tan (x) = CL/67)
ladder AK = 13 + (tan (x) * 67)
ladder BK = 67 + (tan (z) * 13)
tan (x) = (tan (x) * 67)/(tan (z) * 13)
tan (z) = (tan (x) * (tan (x) *67))/13
And then differentiate the last formula?
It's been too long for me since i had this kind of math :(
Sorry that i couldn't help you more
the flea
02-27-2012, 01:10 PM
I didn't really find a solution, but maybe i can help you get some ideas.
http://i39.tinypic.com/23to0ms.jpg
So what i was thinking is if:
ladder DL = tan (z) * 13 ( tan (z) = DL/13)
ladder CL = tan (x) * 67 ( tan (x) = CL/67)
ladder AK = 13 + (tan (x) * 67)
ladder BK = 67 + (tan (z) * 13)
tan (x) = (tan (x) * 67)/(tan (z) * 13)
tan (z) = (tan (x) * (tan (x) *67))/13
And then differentiate the last formula?
It's been too long for me since i had this kind of math :(
Sorry that i couldn't help you more
thanks for the help however I still dont understand what to do :duh:
if it helps anyone trying to solve this the value of x is 30.06765413 and the length AB is 103.3647197 I got the answers because I ran out of time however I can re attempt the question it will just give me different numbers at the start so I am still looking for the solution to this.
okokokok
02-27-2012, 01:13 PM
thanks for the help however I still dont understand what to do :duh:
if it helps anyone trying to solve this the value of x is 30.06765413 and the length AB is 103.3647197 I got the answers because I ran out of time however I can re attempt the question it will just give me different numbers at the start so I am still looking for the solution to this.
Well what i tried to do is if u could get Angle z then you would be able to get DL. Since AB splits a 90 degree corner into z and x' (z-angle) you could also get CL. Then if u have those 2 u know what AK and BK is. Then simply use tan(x) = AK/BK and u would have had AB. But maybe i'm completely wrong...
the flea
02-27-2012, 05:22 PM
just got home from class now. Still not got any further, I hate this type of question pretty much all other types of math im ok with
okokokok
02-27-2012, 05:32 PM
True this is some though math.. :(
Bam Bam
02-28-2012, 08:07 PM
AB = 13/sin(x) + 67/sin(90 - x)
This is based on the assumption that the ladder must contact the corner and that neither 0 or 90 degrees is important.
Differentiate and find x where length is minimum. (Slope = 0)
On my calculator it would seem L_max is 103.36ft and x is 30.07
I can post more when Im not doing so from a phone.
-Bam Bam
the flea
02-28-2012, 08:36 PM
AB = 13/sin(x) + 67/sin(90 - x)
This is based on the assumption that the ladder must contact the corner and that neither 0 or 90 degrees is important.
Differentiate and find x where length is minimum. (Slope = 0)
On my calculator it would seem L_max is 103.36ft and x is 30.07
I can post more when Im not doing so from a phone.
-Bam Bam
yes that's the correct way to do it. My lecturer went through it in our lesson today since the deadline was last night. Well done on solving it I found it really hard
Bam Bam
02-28-2012, 09:35 PM
Sorry I didn't get it done earlier :(
One of my professors today was boring me so I did your problem from memory.
I looked at it last night though and had no idea how to do it then lol :P
-Bam Bam
On my calculator it would seem L_max is 103.36ft and x is 30.07
Got those answers as well using Lmax = (a^(2/3) + b^(2/3) )^(3/2) while phi(Ao) = 0.
I didn't really find a solution, but maybe i can help you get some ideas.
http://i39.tinypic.com/23to0ms.jpg
So what i was thinking is if:
ladder DL = tan (z) * 13 ( tan (z) = DL/13)
ladder CL = tan (x) * 67 ( tan (x) = CL/67)
ladder AK = 13 + (tan (x) * 67)
ladder BK = 67 + (tan (z) * 13)
tan (x) = (tan (x) * 67)/(tan (z) * 13)
tan (z) = (tan (x) * (tan (x) *67))/13
And then differentiate the last formula?
It's been too long for me since i had this kind of math :(
Sorry that i couldn't help you more
tan (y) = sin(y)/cos(y)
and x = 90-z, so tan(z) = sin(90-x)/cos(90-x) = cos(x)/sin(x) = 1/tan(x)
the angle z depends on angle x, you wouldn't get that far with it. you need to express length of ladder as function of x then you can derivate
not to mention that the last two lines don't come out of first four lines.
here is how you do that:
n * sin(x) = a
m * cos(x) = b
n = a/sin(x)
m = b/cos(x)
so length as f(x) = n + m = a/sin(x) + b/cos(x)
derivate of 1/t(x) = -t'(x) / t^2(x)
so a/sin(x) derivate is -acos(x)/(sin(x)^2) and b/cos(x) derivate is bsin(x)/(cos(x)^2)
so you are looking for 0= a cos(x)^3 - b sin(x)^3, or after few adjustments:
tan(x)^3 = a/b
tan(x) = (a/b)^(1/3) (cubic root)
therefore x = approx 30.06
Edit: oops forgot the little drawing with m and n on it
http://oi46.tinypic.com/dp77rq.jpg
doorsftw
05-25-2012, 07:49 PM
i cant see the problem...
Pascal
05-26-2012, 11:27 PM
Try the cos-rule.
putonajonny
05-28-2012, 04:57 PM
Angle = 30.06765414 Degrees
AB = 103.3647197 units
Will take a photo of my working in a sec and upload it, but it seems to be consistent with other people and your answer
http://i.imgur.com/p0y55.jpg
Also post more things like this, maths problems are really fun imo
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