Funny math problem to calculate the Pin Number for a credit card.
08-10-2021, 05:31 AM (This post was last modified: 08-10-2021 05:43 AM by Steve Simpkin.)
Post: #1 Steve Simpkin Senior Member Posts: 691 Joined: Dec 2013
Funny math problem to calculate the Pin Number for a credit card.
This cute math problem to calculate the Pin Number for a credit card popped up on a post in the Facebook HP Calculator Fan Club group recently. Apologies if it has been posted before. 08-10-2021, 05:34 AM
Post: #2 Steve Simpkin Senior Member Posts: 691 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
Here is the solution to this problem using the free HP Prime Lite app. 08-10-2021, 05:41 AM
Post: #3 Steve Simpkin Senior Member Posts: 691 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
Here is the solution using a Casio fx-991EX.
I had to set the upper limit to .9999999999 (stored in Y) to avoid an error on this model.
At about \$18 USD, this is probably the least expensive physical calculator currently available new that can solve this problem. 08-11-2021, 03:40 PM (This post was last modified: 08-12-2021 10:45 AM by Albert Chan.)
Post: #4
 Albert Chan Senior Member Posts: 1,786 Joined: Jul 2018
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-10-2021 05:41 AM)Steve Simpkin Wrote:  Here is the solution using a Casio fx-991EX.
I had to set the upper limit to .9999999999 (stored in Y) to avoid an error on this model.

FYI, integrand singularity (at x=1) is removable.

(3x^3-x^2+2x-4) / √(x^2-3x+2)
= (x-1)*(3*x^2+2*x+4) / √((x-1)*(x-2))
= -(1-x)*(3*x^2+2*x+4) / √((1-x)*(2-x)) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // integral limits 0 to 1, √(1-x), √(2-x) both real
= -(3*x^2+2*x+4) * √(1-1/(2-x))

XCAS> -∫((3*x^2+2*x+4) * √(1-1/(2-x)), x=0..1)

(-202*sqrt(2) + 135*ln(2*sqrt(2)+3))/16 ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // ≈ -2.98126694401

---

Proof: assume integral have the form f*√g

(f*√g)' = f'*√g + f/(2√g)*g' = (f'*g + f*g'/2) / √g

XCAS> f := a*x^2+b*x+c ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // integrand numerator is cubic, thus quadratic f
XCAS> g := x^2 - 3x + 2 ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // integrand denominator = ﻿√g
XCAS> coefs := e2r(f'*g + f*g'/2)

[3*a, (-15*a+4*b)/2, (8*a-9*b+2*c)/2, (4*b-3*c)/2]

Ignore constant term for now, we match 3 coefs with 3 unknown.

XCAS> a, b, c := 1, 13/4, 101/8
XCAS> coefs

[3, -1, 2, -199/16] ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // = [3, -1, 2, -4] - [0, 0, 0, 135/16]

I1 = preval(f*√g, 0, 1) = subst(-f*√g, x=0) = -101/8*√(2)

I2 = ∫(1/√(x^2-3x+2), x=0..1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // let y = 2*(x-3/2)=2x-3, dy = 2 dx
﻿ ﻿ ﻿ ﻿ = ∫(1/√(y^2-1), y=-3 .. -1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // ∫(1/√(y^2-1), y) = -ln(abs(-y+sqrt(y^2-1)))
﻿ ﻿ ﻿ ﻿ = ln(2*√(2)+3)

I = I1 + 135/16 * I2 = -101/8*√(2) + 135/16*ln(2*√(2)+3)
08-12-2021, 07:54 AM
Post: #5
 Werner Senior Member Posts: 685 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
You never cease to amaze me, Albert!
Took me only three reads to understand this time ;-)
Nice trick.

Cheers, Werner
08-12-2021, 03:05 PM (This post was last modified: 08-12-2021 03:52 PM by Albert Chan.)
Post: #6
 Albert Chan Senior Member Posts: 1,786 Joined: Jul 2018
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-11-2021 03:40 PM)Albert Chan Wrote:  I2 = ∫(1/√(x^2-3x+2), x=0..1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // let y = 2*(x-3/2)=2x-3, dy = 2 dx
﻿ ﻿ ﻿ ﻿ = ∫(1/√(y^2-1), y=-3 .. -1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ // ∫(1/√(y^2-1), y) = -ln(abs(-y+sqrt(y^2-1)))
﻿ ﻿ ﻿ ﻿ = ln(2*√(2)+3)

Another way is to map y=2*x-3 for the full integral, then solve by integration by parts
Note that above quoted expression, map from x to y, numerator of 1 stayed 1.

XCAS> e2r(r2e([3,-1,2,-4], (y+3)/2), y) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ → [3/8, 25/8, 77/8, 55/8]

I = ∫((3y^3 + 25y^2 + 77y + 55) / √(y^2-1), y=-3 .. -1) / 8

Y0 = ∫(1/√(y^2-1),y) = -ln(abs(-y+√(y^2-1)))
Y1 = ∫(y/√(y^2-1),y) = ∫d(√(y^2-1)) = √(y^2-1)

Y2 = ∫(y^2/√(y^2-1),y) = ∫(y*d(Y1)) = y*Y1 - (Y2-Y0)
2*Y2 = y*Y1 + Y0

Y3 = ∫(y^3/√(y^2-1),y) = ∫(y^2*d(Y1)) = y^2*Y1 - 2*(Y3-Y1)
3*Y3 = (y^2+2)*Y1

3*Y3 + 25*Y2 + 77*Y1 + 55*Y0
= (y^2+2)*Y1 + 25/2*(y*Y1+Y0) + 77*Y1 + 55*Y0
= (y^2+25/2*y+79)*Y1 + 135/2*Y0

I = preval((y^2+25/2*y+79)*√(y^2-1) + 135/2*(-ln(abs(-y+√(y^2-1)))), -3, -1, y) / 8
﻿ ﻿ = -101/8*√(2) + 135/16*ln(2*√(2)+3)
﻿ ﻿ ≈ -2.98126694401
08-12-2021, 11:41 PM
Post: #7 Don Shepherd Senior Member Posts: 745 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
I guess I just don't see the point. A credit card that requres a PIN number either comes with a pre-assigned PIN or lets you choose the PIN; most of mine let you choose. A calculated PIN would seem unnecessary in both cases.
08-13-2021, 12:02 AM
Post: #8 Steve Simpkin Senior Member Posts: 691 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-12-2021 11:41 PM)Don Shepherd Wrote:  I guess I just don't see the point. A credit card that requres a PIN number either comes with a pre-assigned PIN or lets you choose the PIN; most of mine let you choose. A calculated PIN would seem unnecessary in both cases.

I think the humor here is that the person writing the letter and being generous with their credit card (probably uncharacteristically for them) assumes their partner will not be able to solve the math problem and get the PIN number (probably 2981). I showed this to my wife and she laughed and then said "Ass".
08-13-2021, 05:30 AM
Post: #9 Namir Senior Member Posts: 822 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
Solve on the HP15C gives pretty much the same value for the integral between 0 and 0.99999.
08-13-2021, 07:52 AM
Post: #10 OlidaBel Member Posts: 56 Joined: Mar 2021
RE: Funny math problem to calculate the Pin Number for a credit card.
Hi,
Could someone test it on a real HP-42S ?
Because I've just tested in its swiss clone and I don't get error with [0, 1] boundaries ;-)

---
HP 48GX, Prime G2, 50G. A long time ago : 11C, 15C, 28C, 28S. SwissMicros DM42, DM15L
08-13-2021, 08:57 AM
Post: #11
 Werner Senior Member Posts: 685 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
It is well-known that the numerical integration in, I think, all its HP implementations (from 34C onwards) never evaluates the endpoints. Free42 and, by extension, the DM42 are such faithful simulators that they of course mimic that, too.
But I did try it out on a real 42S of course. No problem whatsoever.

Code:
00 { 41-Byte Prgm } 01▸LBL "FX" 02 MVAR "X" 03 RCL "X" 04 ENTER 05 STO ST Z 06 3 07 × 08 1 09 - 10 × 11 2 12 + 13 × 14 4 15 - 16 X<>Y 17 X^2 18 3 19 LASTX 20 × 21 - 22 2 23 + 24 SQRT 25 ÷ 26 END

With ACC= 0.0001 I get -2.98125065973

Cheers, Werner
08-13-2021, 05:26 PM
Post: #12
 Albert Chan Senior Member Posts: 1,786 Joined: Jul 2018
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-12-2021 03:05 PM)Albert Chan Wrote:  I = ∫((3y^3 + 25y^2 + 77y + 55) / √(y^2-1), y=-3 .. -1) / 8

Another way, by regroup terms. (odd and even powers of y)

diff(y*√(y^2-1), y) = (2*y^2-1) / √(y^2-1))

I = ∫((3/8*(y^2-1)+10)*y/√(y^2-1), y=-3 .. -1) + ∫((25/16*(2*y^2-1)+135/16)/√(y^2-1), y=-3 .. -1)

For first term, let z = y^2-1, dz = 2y dy:

∫((3/8*(y^2-1)+10)*y/√(y^2-1), y) ﻿= ∫(3/16*√z+5/√z, z) = 1/8*z^(3/2) + 10*√z

I = preval((z/8+10)*√z,8,0,z) + preval(25/16*(y*√(y^2-1)) + 135/16*(-ln(abs(-y+√(y^2-1)))),-3,-1,y)
﻿ ﻿ = -22*√(2) + (150*√(2)+135*ln(2*√(2)+3))/16
﻿ ﻿ = -101/8*√(2) + 135/16*ln(2*√(2)+3)
08-13-2021, 08:00 PM
Post: #13
 toml_12953 Senior Member Posts: 1,845 Joined: Dec 2013
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-10-2021 05:31 AM)Steve Simpkin Wrote:  This cute math problem to calculate the Pin Number for a credit card popped up on a post in the Facebook HP Calculator Fan Club group recently. Apologies if it has been posted before.

I see an answer with a lot of decimals but what's the PIN?

Tom L
Cui bono?
08-18-2021, 12:19 AM
Post: #14
 johanw Member Posts: 167 Joined: Nov 2019
RE: Funny math problem to calculate the Pin Number for a credit card.
(08-13-2021 08:00 PM)toml_12953 Wrote:  I see an answer with a lot of decimals but what's the PIN?
The first 4 decimals?
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