## Basic math problem I think I'm getting wrong...

For the discussion of math. Duh.

Moderators: gmalivuk, Moderators General, Prelates

neoliminal
Posts: 626
Joined: Wed Feb 18, 2009 6:39 pm UTC

### Basic math problem I think I'm getting wrong...

I need the square meters of two trapezoids.

A=20.96
B=8.99
C=12.86
D=8.24

I'm getting 128 m2

The other is

A=8.99
B=7.60
C=8.56
D=7.99

I'm getting 40 m2

What am I doing wrong... or are these right? I can't see how they can be right.
http://www.amazon.com/dp/B0073YYXRC
Read My Book. Cost less than coffee. Will probably keep you awake longer.
[hint, scary!]

jaap
Posts: 2094
Joined: Fri Jul 06, 2007 7:06 am UTC
Contact:

### Re: Basic math problem I think I'm getting wrong...

I've only checked the first one, and get 128.025618..., so you seem to have that one right (assuming the lengths you gave are in metres).

OverBored
Posts: 284
Joined: Mon Dec 10, 2007 7:39 pm UTC

### Re: Basic math problem I think I'm getting wrong...

I agree with Jaap for the first one, but I get a different answer from you for the second (I'm not sure I believe my answer though). I get 28.79827...

Edit: Incidentally, I am using the formula given here
G4!!

Grob FTW,

Hello. Smithers. You're. Quite good. At. Turning. Me. On.

jaap
Posts: 2094
Joined: Fri Jul 06, 2007 7:06 am UTC
Contact:

### Re: Basic math problem I think I'm getting wrong...

OverBored wrote:I agree with Jaap for the first one, but I get a different answer from you for the second (I'm not sure I believe my answer though). I get 28.79827...

I get the same answer for the second one, too.

neoliminal
Posts: 626
Joined: Wed Feb 18, 2009 6:39 pm UTC

### Re: Basic math problem I think I'm getting wrong...

Ah, I see how 28.796 is right, but if I cut the trapezoid to its smallest sides and make that a rectangle of

8.56 and 7.60 the area is 65.056... so it can't be possible that 28 square meters is right. That's a little over a 5m by 5m square.
http://www.amazon.com/dp/B0073YYXRC
Read My Book. Cost less than coffee. Will probably keep you awake longer.
[hint, scary!]

neoliminal
Posts: 626
Joined: Wed Feb 18, 2009 6:39 pm UTC

### Re: Basic math problem I think I'm getting wrong...

http://calculator.tutorvista.com/math/3 ... ator.html#

Found this. Actual answer to #2 is 66.69m2
http://www.amazon.com/dp/B0073YYXRC
Read My Book. Cost less than coffee. Will probably keep you awake longer.
[hint, scary!]

jaap
Posts: 2094
Joined: Fri Jul 06, 2007 7:06 am UTC
Contact:

### Re: Basic math problem I think I'm getting wrong...

neoliminal wrote:http://calculator.tutorvista.com/math/381/area-of-isosceles-trapezoid-calculator.html#

Found this. Actual answer to #2 is 66.69m2

What are you using as the height?

Your trapezoid #2 has:
A=8.99
B=7.60
C=8.56
D=7.99
Notice that A-C = 0.43, and D-B = 0.39, which is very close. The only way this can be is if it is very skewed, so not much like a square or rectangle at all but more like a very squashed parallelogram. Its height is around 3.28.

curtis95112
Posts: 638
Joined: Thu Jan 27, 2011 5:23 pm UTC

### Re: Basic math problem I think I'm getting wrong...

Wolfram Alpha tells me that the second is not a possible trapezoid.
http://www.wolframalpha.com/input/?i=area+of+trapezoid+with+side+lengths+8.99%2C+7.6%2C+8.56%2C+7.99
Mighty Jalapeno wrote:
Tyndmyr wrote:
Роберт wrote:Sure, but at least they hit the intended target that time.

Well, if you shoot enough people, you're bound to get the right one eventually.

Thats the best description of the USA ever.

jaap
Posts: 2094
Joined: Fri Jul 06, 2007 7:06 am UTC
Contact:

### Re: Basic math problem I think I'm getting wrong...

curtis95112 wrote:Wolfram Alpha tells me that the second is not a possible trapezoid.
http://www.wolframalpha.com/input/?i=area+of+trapezoid+with+side+lengths+8.99%2C+7.6%2C+8.56%2C+7.99

It seems to be unable to give an answer regardless of the values. For example it has no answer for 1, 3, 2, 3 which is pretty easily seen to be valid due to its symmetry. Or try 1, sqrt(2), 2, 1, which is another obviously valid one.

Meem1029
Posts: 379
Joined: Wed Jul 21, 2010 1:11 am UTC

### Re: Basic math problem I think I'm getting wrong...

It also says there is no trapezoid with side lengths of 1,1,1,1. I think we can all agree that unless it is using some obscure and weird definition of trapezoid it's wrong.
cjmcjmcjmcjm wrote:If it can't be done in an 80x24 terminal, it's not worth doing

Nitrodon
Posts: 497
Joined: Wed Dec 19, 2007 5:11 pm UTC

### Re: Basic math problem I think I'm getting wrong...

Meem1029 wrote:It also says there is no trapezoid with side lengths of 1,1,1,1. I think we can all agree that unless it is using some obscure and weird definition of trapezoid it's wrong.

A trapezoid must have exactly one pair of parallel sides. A quadrilateral with side lengths 1, 1, 1, 1 is a rhombus, and thus not a trapezoid.

That said, asking it about a trapezoid with side lengths 1, 2, 1, 1 makes it assume that the bases are unknown variables "a" and "b". I don't get this at all.

gmalivuk
GNU Terry Pratchett
Posts: 26765
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Basic math problem I think I'm getting wrong...

Nitrodon wrote:A trapezoid must have exactly one pair of parallel sides. A quadrilateral with side lengths 1, 1, 1, 1 is a rhombus, and thus not a trapezoid.
I think you'll find that, as most people use it (especially for higher math like calculus, where the trapezoid approximation is a thing), the inclusive definition is used: a trapezoid has *at least* one pair of parallel sides.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

SerialTroll
Posts: 82
Joined: Mon Dec 20, 2010 5:28 am UTC

### Re: Basic math problem I think I'm getting wrong...

So many people using tools on the internet instead of doing the math. I am disappoint.