Who's Good At Math?

6stringmason

I have an isoceles(SP?) triangle I need to find the area of. Its an area going into the roof line. Ten foot from across bottom with a height of 16'. If it were laid upright.

The 16' is the soffit line running into the roof. The straight corner down is 10'. LOL! Make sense? Im ok at regular math and the majority of algebra, but tryg is for suckers. Any help would be cool.

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Mike Finley

http://mathworld.wolfram.com/IsoscelesTriangle.html

PipeGuy

try this
http://www.ajdesigner.com/phptriangl...le_area_bh.php

isosceles is at the bottom

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Teetorbilt

10 X 16 = 160 / 2 = 80

1/2B X H (old school)

5 X 16 = 80

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6stringmason

Thanks for the quick replies guys. I know that a regular triangle is 1/2BxH but I was looking at the equation for the isoceles and thought no way am I going to get this crap lol.

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go dart

1^44~///x1.6% zero niner. opps nevermind!

RowdyRed94

Congrats on actually knowing some math. That's a rare thing these days. My question is does it work for an obtuse triangle, too. I'd guess yes.

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erik edlund

Yes, but does it work for an obtuse moderator?!

(Sorry, Teetor, just a ribbing for 'ya, fine answer, sir).

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dantheman

<i>My question is does it work for an obtuse triangle, too. I'd guess yes.</i>

Works for that, too. I could give you a rigorous proof if you want it, but it comes down to the fact that the obtuse triangle can be split into two right triangles.

dirt diggler

try to look at the triangle as "half" of a rectangle (cut diagonally)

dantheman

Hard to visualize that without breaking it up into right triangles first, though. Your explanation is better.

Newton

area is 90.977593395297 using one of the links

base is 10
height is 16
side is 18.87
plug in numbers to formula

Bradracer18

Finding the area of an isosceles triange is NOT the same as a right triangle. They differ, hard to explain.....but they do.


I got the same answer as Newton, doing it my way......try using those numbers, .5bh does not give you the same answer(gives you 80).

Double-A

Your length for the equal sides is wrong. Its not 18.87', its 16.76'.

The formula Area=1/2b*h works for all triangles but there is a margin of error introduced into it for some types, the isosceles is one of those. The error is insignificant. See the Google SketchUp images below.

When worked with 1/2bh ... 0.5*10*16 = 80 square feet. I had to solve for one of the equal sides.

The other is using the more complicated and more accurate formula shown in post #3. The answer using the more complicated formula
Area=b[SQRT(4*a^2-b^2)/4] comes out to 79.9983 square feet, or (79' 11 31/32")^2. Takes more room than that to land a fly.

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Bradracer18

Well......I was wondering about that.......couldn't remember how they differed, but my numbers(well his) said it differed.

You'd think I'd know this stuff, going into engineering and all......have been through many geometry, trig, calc, and differential equations classes.....probably too much math, can't remember any now. lol.'

Good job AA....nice visual too!!!

Double-A

I'd really like to have seen a picture of the area 6string was referring to. I'd hate to work on a roof with the pitch my drawing has.

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mikesewell

Double-A,
Nice try. Wrong answer. The correct answer is 80 Ft^2

In effect, what you are telling us, is that a 5 Ft x 16 Ft rectangle has an area of 79.9983 Ft^2. Not right.

You went over the river and through the woods to get to grandma's house. She lives next door.

What Teetor said. A = 1/2bh

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Double-A

No, I was pointing out that while there was more than one way to compute the area, as done in the first few posts, its faster and easier to use the A=1/2bh formula. Its the one I use.

My point was this. You can go through all the gyrations of using the more complex formulas, which are all extrapolations of the Pythagorean theorem, or you can save time and headaches and just use Area = 1/2hb.

As for the wrong answer, I beg to differ, but wouldn't be surprised. I was only using 4 significant digits and the online calculator may have some issues with it, accounting for the 1/32"^2 difference in answers. Its actually less than that.

In the real world, we'd have rounded off long ago and be at 80'^2.

I agree 80'^2 is the answer to use. And it would be a parallelogram, not a rectangle. This was an isosceles triangle we started with, not a right triangle.

I'd still hate to work on that roof.

EDIT: After rereading what you wrote, you're right. The 79.whatever is wrong. But, blame the formula, not me. I use the KIS system in the field. ( I don't use the KISS system, one added step, not worth the effort.)

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mikesewell

The formula that you used is fine, but it's not the easiest one to use for this application. If carried to completion (an infinite number of decimal places), it yields the correct answer (80 Ft^2). There's nothing to round off. More digits would just be 80.00000000000000... Ft^2.

If the height were the unknown quantity, then A = 1/2 bh wouldn't work until you found the height, but make no mistake about it, A = 1/2 bh IS trigonometry, and is TRUE, it is not just a cheap and handy parlor trick that's used to arrive at a close approximation of the truth.

I didn't mean to pick on you, I just wanted to clear things up for the rigorous proof impaired, and the infinity impaired.

The easiest way to look at it this problem is as two right triangles, back to back. Flip one around and you have a rectangle.

I hope that I have helped to clear things up, and not just muddied the waters further.

Also:
Hat's off to dantheman for knowing the importance of rigorous proof. Nice offer dan; do you really want to work that hard, or are you going to give us a link to the rigorous proof?

Best regards,

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Teetorbilt

Anybody know how to get advanced math symbols onto this site?

You can always play around here. http://www.engineersedge.com/calcula...ution_menu.htm

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You can't solve you're problems with the same level of thinking that created the problems.

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