Math problem where graphing calculator may slow you down - part II.
09-12-2014, 12:43 PM (This post was last modified: 09-12-2014 12:43 PM by CR Haeger.)
Post: #1
 CR Haeger Member Posts: 275 Joined: Dec 2013
Math problem where graphing calculator may slow you down - part II.
Hello - here is a problem that should be fairly easily solvable by hand and with a trig table:

Assume f(x) = max(sin(x), cos(x)) using radians. If the area under the curve f(x) = 4.0 and the lower limit of x = 0.0, what is the upper limit of integration?

What do your machines and/or brains come up with?

Note - I got this to work on the HP Prime using the AREA() function in Function APP, but had problems using solve() or fsolve() properly. Num-Solv on a TI36x had to think about this a looong time.
09-12-2014, 04:33 PM (This post was last modified: 09-12-2014 04:54 PM by Marcus von Cube.)
Post: #2
 Marcus von Cube Senior Member Posts: 760 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(09-12-2014 12:43 PM)CR Haeger Wrote:  If the area under the curve f(x) = 4.0
Is the area positive if the function is negative? Or are we just talking about the definite integral?

EDIT: If the area is always positive, the answer 2π. I split the interval at π/4, π, π+π/4, π+π/2, 2π. All the square roots cancel each other.

Marcus von Cube
Wehrheim, Germany
http://www.mvcsys.de
http://wp34s.sf.net
http://mvcsys.de/doc/basic-compare.html
09-12-2014, 05:22 PM
Post: #3
 CR Haeger Member Posts: 275 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(09-12-2014 04:33 PM)Marcus von Cube Wrote:
(09-12-2014 12:43 PM)CR Haeger Wrote:  If the area under the curve f(x) = 4.0
Is the area positive if the function is negative? Or are we just talking about the definite integral?

EDIT: If the area is always positive, the answer 2π. I split the interval at π/4, π, π+π/4, π+π/2, 2π. All the square roots cancel each other.

Hi Marcus, I should have stated that its a definite integral where area is negative when f(x) is negative. Your solution and abs(f(x)) are interesting too - thanks!
09-13-2014, 04:54 AM
Post: #4
 Thomas Klemm Senior Member Posts: 1,447 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
My solution by hand: $$2\pi+\arccos(3\sqrt{2}-4)$$
I used the HP-15C with this program to calculate the function:
Code:
LBL A 1 ->R x<y x<>y RTN
And then another program to calculate the integral which I finally used with the solver.

Cheers
Thomas
09-13-2014, 09:12 PM
Post: #5
 Bunuel66 On Vacation Posts: 29 Joined: Jan 2014
RE: Math problem where graphing calculator may slow you down - part II.
By hand and a bit of brain....
f(x) is periodic with a 2pi period.
from 0 to pi/4 f(x)=cos(x)
from pi/4 to 5pi/4 f(x)=sin(x)
from 5pi/4 to 2pi f(x)=cos(x)

Integrating is straightforward and gives 4/sqrt(2) over 0 to 2pi
going up to 9pi/4 gives 5/sqrt(2).
Then the value searched for is between 9pi/4 and 9pi/4+pi, in other words on the second interval of the definition of f(x).
This equivalent to search for X with: sum from pi/4 to X of sin(x) dx=4-5/sqrt(2).
Integrating analytically and solving lead to: X=acos(6/sqrt(2)-4)
The solution is then 9pi/4+X-pi/4~7.6089

Nothing more necessary than any standard scientific calculator...
BTW, I'm cheating slightly, I checked the value of the integral with my own python software ;-)
which uses a HP like algorithm....
09-14-2014, 02:18 PM
Post: #6
 Thomas Klemm Senior Member Posts: 1,447 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(09-13-2014 09:12 PM)Bunuel66 Wrote:  Integrating is straightforward and gives 4/sqrt(2) over 0 to 2pi

We should make sure that the integral is always < 4. Thus we calculate it up to the first zero of the function $$f(x)$$ which occurs at $$x=\pi$$:
$\int_{0}^{\pi}\max(\sin(x),\cos(x))dx=1+\sqrt{2}\approx 2.41421$

Cheers
Thomas
09-14-2014, 07:22 PM
Post: #7
 Bunuel66 On Vacation Posts: 29 Joined: Jan 2014
RE: Math problem where graphing calculator may slow you down - part II.
Actually I made the check for the first maximum, I wanted just not to be too long with the explanations, just giving the general idea.

Regards
11-14-2014, 10:01 PM (This post was last modified: 11-14-2014 10:40 PM by Gilles.)
Post: #8
 Gilles Member Posts: 167 Joined: Oct 2014
RE: Math problem where graphing calculator may slow you down - part II.
I used the numeric solver of the 50G in FIX 4

FIX 6 takes more time and returns
X: 7.608894

This help to get the exact answer with the 50G CAS :
'X=2*PI+ACOS(3*SQRT(2)-4)'
11-16-2014, 04:06 PM (This post was last modified: 11-16-2014 04:21 PM by CR Haeger.)
Post: #9
 CR Haeger Member Posts: 275 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
Thanks Giles,

Has anyone tried solving this using the HP Prime CAS, Home or Function APP? It seems to me there are only a couple of ways to find a numeric solution. I have not found any exact solution using the device either.

Screenshot with CAS settings Exact, Complex and Use i unchecked.

Graphically, it worked out pretty well, once I figured out AREA() syntax. Note that using integral from template did not work for me.

11-16-2014, 08:40 PM (This post was last modified: 11-16-2014 08:45 PM by Gilles.)
Post: #10
 Gilles Member Posts: 167 Joined: Oct 2014
RE: Math problem where graphing calculator may slow you down - part II.
Hi CR Haeger, I tried with the Prime both in CAS and with the SOLVE APPS without success.

I agree with you about the plotter of the Prime : just fabulous !
I like very much to zoom instantly with 2 fingers
11-28-2014, 06:17 PM
Post: #11
 lrdheat Senior Member Posts: 574 Joined: Feb 2014
RE: Math problem where graphing calculator may slow you down - part II.
I'm amazed at the TI 36 Pro. It took 30+ minutes (in numsolve), but it came up with the result!
11-29-2014, 12:49 AM
Post: #12
 lrdheat Senior Member Posts: 574 Joined: Feb 2014
RE: Math problem where graphing calculator may slow you down - part II.
I sped things up to under 15 minutes by doing a couple of experimental integrations to get an idea of where the integral would be ~4, then a quick table to zoom in on an appropriate guess, and then enter a good guess into numsolve.
11-29-2014, 03:31 AM
Post: #13
 lrdheat Senior Member Posts: 574 Joined: Feb 2014
RE: Math problem where graphing calculator may slow you down - part II.
...which leads me to...how to accomplish this on the WP-34S? How do I solve for an integral ?
11-29-2014, 09:30 AM
Post: #14
 Thomas Klemm Senior Member Posts: 1,447 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(11-29-2014 03:31 AM)lrdheat Wrote:  how to accomplish this on the WP-34S?

It's similar to what you do using the HP-15C:
Code:
LBL'FX' 1 ->REC MAX RTN

Code:
LBL'IGR' 0 x<>y ∫'FX' 4 - RTN

FIX 4
7.6
7.61
SLV'IGR'

7.6089

Cheers
Thomas
11-29-2014, 01:52 PM (This post was last modified: 11-29-2014 01:54 PM by CR Haeger.)
Post: #15
 CR Haeger Member Posts: 275 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(11-29-2014 12:49 AM)lrdheat Wrote:  I sped things up to under 15 minutes by doing a couple of experimental integrations to get an idea of where the integral would be ~4, then a quick table to zoom in on an appropriate guess, and then enter a good guess into numsolve.

Try turning off pretty print mode and setting the integrals resolution to say 0.1. May speed things up.

I agree - the TI36x is pretty straightforward and capable for these type if solver problems.
11-30-2014, 01:33 AM
Post: #16
 lrdheat Senior Member Posts: 574 Joined: Feb 2014
RE: Math problem where graphing calculator may slow you down - part II.
Thanks Thomas,

It works...couldn't remember how to have an integral in solve. Rather slow on this sort of problem!
11-30-2014, 12:16 PM
Post: #17
 Thomas Klemm Senior Member Posts: 1,447 Joined: Dec 2013
RE: Math problem where graphing calculator may slow you down - part II.
(11-30-2014 01:33 AM)lrdheat Wrote:  Rather slow on this sort of problem!

For this specific problem the derivative is trivial:

\begin{align} F(x)&=\int_{0}^{x}f(t)dt-4 \\ F'(x)&=f(x) \\ \end{align}

Thus we can use Newton's method:
\begin{align} x'&=x-\frac{F(x)}{F'(x)} \\ &=x-\frac{\int_{0}^{x}f(t)dt-4}{f(x)} \\ &=x+\frac{\int_{x}^{0}f(t)dt+4}{f(x)} \\ \end{align}

Instead of starting the integration from $$0$$ over and over again we can reuse the result of $$F(x)$$ from the previous loop:
$F(x')=F(x)+\int_{x}^{x'}f(t)dt$
This value is saved in register 01.

Code:
LBL'FX' # 001 →REC MAX RTN

Code:
LBL'NWT' # 004 STO 01 CLx STO 00 R↓ LBL 00 ENTER↑ x<> 00 ∫'FX' RCL+ 01 STO 01 RCL 00 XEQ'FX' / x≈0? SKIP 002 RCL+ 00 GTO 00 RCL 00 END

This will speed up the calculation.

Cheers
Thomas
11-30-2014, 07:04 PM
Post: #18
 lrdheat Senior Member Posts: 574 Joined: Feb 2014
RE: Math problem where graphing calculator may slow you down - part II.
Excellent, clear, concise as usual.
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