Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

I think my and irspow's methods are pretty much the same, just worded differently. We get the same answer, anyway.

Why did the vector cross the road?

It wanted to be normal.

Offline

**irspow****Member**- Registered: 2005-11-24
- Posts: 455

Sorry, I didn't mean to jump in your spot. The wording of this problem is what had me scratching though. I wasn't sure if ganesh wanted the over-all probability of drawing a defective bolt from B or the probability that I (or should I say we?) calculated.

Offline

**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

Don't worry, you're in your own spot. Our spots just happen to look similar.

Anyway, I think we're right. Ganesh told us that the bolt was defective, and wanted to know tha probability of it being from B.

Why did the vector cross the road?

It wanted to be normal.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Both mathsyperson and irspow are correct. Well done.

Problem # k + 90

Find the formula for sum of n terms of

(1*2*3), (2*3*4), (3*4*5), (4*5*6),................. Prove that the formula is correct by Mathematical Induction.

Character is who you are when no one is looking.

Offline

**Ricky****Moderator**- Registered: 2005-12-04
- Posts: 3,791

k+90's formula is:

Anybody want to try their hand at an induction proof, or should I?

"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Ricky is correct. I shall wait for someone to post a proof.

Character is who you are when no one is looking.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Problem # k + 91

Find three consecutive terms in Geometric Progression such that their sum is 21 and the sum of their squares is 189.

Character is who you are when no one is looking.

Offline

**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

Why did the vector cross the road?

It wanted to be normal.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

mathsyperson's solution to problem # k + 91 is correct. I shall post the method if none of the others are able to give a proper solution with the steps.

Problem # k + 92

If x=9 + 4√5, find the value of √x - 1/√x.

Character is who you are when no one is looking.

Offline

**Ricky****Moderator**- Registered: 2005-12-04
- Posts: 3,791

I have the induction proof written up on paper, but I'm a bit too lazy to type it all up now. Maybe later tonight.

"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."

Offline

**irspow****Member**- Registered: 2005-11-24
- Posts: 455

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Ricky and irspow are correct!

Character is who you are when no one is looking.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Problem # k + 93

A right ciruclar cone is cut by two planes parallel to the base, trisecting the altitude. What is the ratio of the volumes of the three parts, the top, middle and the bottom respectively?

Character is who you are when no one is looking.

Offline

**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

Why did the vector cross the road?

It wanted to be normal.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Well done, mathsyperson!

Problem # k + 94

If 1/a + 1/b + 1/c = 1/(a+b+c); a+b+c≠0 and abc≠0, what is the value of (a+b)(b+c)(c+a)?

Character is who you are when no one is looking.

Offline

**irspow****Member**- Registered: 2005-11-24
- Posts: 455

Nice job, Mathsyperson! I spent like twenty minutes to come up with the same answer using the disk method of integration.

Offline

**irspow****Member**- Registered: 2005-11-24
- Posts: 455

This is not my type of problem, but here goes nothing.

*edit

Duh! I forgot to subtract abc from **both** sides of the equation.

*Last edited by irspow (2006-02-19 01:14:16)*

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

irspow, please check the solution you posted again

Character is who you are when no one is looking.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Problem # k + 95

1 man or 2 women or three boys can do a work in 55 days. In how many days can 1 man, 1 woman and 1 boy do the same work?

Character is who you are when no one is looking.

Offline

**krassi_holmz****Real Member**- Registered: 2005-12-02
- Posts: 1,908

So a=-c or a=-b and

*Last edited by krassi_holmz (2006-02-18 22:26:54)*

IPBLE: Increasing Performance By Lowering Expectations.

Offline

**krassi_holmz****Real Member**- Registered: 2005-12-02
- Posts: 1,908

k+95:

1m-1/55 per day

1w-1/110 per day

1b-1/165 per day

t-the numbers of days:

t(1/55+1/110+1/165) = 1;

t(1/30)=1;

t=30 days.

IPBLE: Increasing Performance By Lowering Expectations.

Offline

**irspow****Member**- Registered: 2005-11-24
- Posts: 455

Offline

**krassi_holmz****Real Member**- Registered: 2005-12-02
- Posts: 1,908

Yeah

IPBLE: Increasing Performance By Lowering Expectations.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Character is who you are when no one is looking.

Offline

**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 15,418

Problem # k + 96

Can a majority of the numbers from 1 to a million be represented as the sum of a square and a (non-negative) cube?

Character is who you are when no one is looking.

Offline