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

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

Postby neoliminal » Wed May 16, 2012 7:22 pm UTC

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

Postby jaap » Wed May 16, 2012 7:33 pm UTC

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).

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

Postby OverBored » Wed May 16, 2012 7:37 pm UTC

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

Postby jaap » Wed May 16, 2012 7:49 pm UTC

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.

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

Postby neoliminal » Wed May 16, 2012 7:58 pm UTC

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

Postby neoliminal » Wed May 16, 2012 8:47 pm UTC

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

Found this. Actual answer to #2 is 66.69m2
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Re: Basic math problem I think I'm getting wrong...

Postby jaap » Thu May 17, 2012 1:42 am UTC

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.

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

Postby curtis95112 » Thu May 17, 2012 1:40 pm UTC

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

Postby jaap » Thu May 17, 2012 2:55 pm UTC

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.

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

Postby Meem1029 » Thu May 17, 2012 6:56 pm UTC

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

Postby Nitrodon » Thu May 17, 2012 9:04 pm UTC

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.

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

Postby gmalivuk » Fri May 18, 2012 1:27 am UTC

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

Postby SerialTroll » Fri May 18, 2012 2:16 pm UTC

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


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