# What is this shape called?

1. Jun 21, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
2. Jun 21, 2011

### micromass

Staff Emeritus
Hi FP!

A pentagon is a good name for this. A pentagon is just a thingy with 5 corners and 5 sides.
It isn't a regular pentagon though, for that we would require all sides and all angles to be equal.

3. Jun 21, 2011

### Pengwuino

Giant diamond.

4. Jun 21, 2011

### Femme_physics

Hi micromass! :) (I love this new "hi" thing lol!)

The reason I'm asking is that I'm trying to find X here

http://img10.imageshack.us/img10/3268/yjjjj.jpg [Broken]

And I know the area is 3375 cm squared. I was hoping the relate the formula for area for this shape to X, but I'm unable to find its formula on my formula page (I can't find the shape, even)

Oh, and there's no "diamond" there :P

Last edited by a moderator: May 5, 2017
5. Jun 21, 2011

### micromass

Staff Emeritus
Ah, I see. Well, I guess the area of a parallellogram is on your formula page?

Well, let's try to find a formula for the giant diamond (I love that name )

The trick is to subdivide the giant diamond into a parallellogram and a triangle. Just take the two mid-vertices (not the vertices on top and not the vertex on the bottom) and connect them with a line. Now the top is a parallellogram and the bottom is a triangle. Can you find the area of the parallellogram and the triangle?

6. Jun 21, 2011

### eumyang

I think you mean a trapezoid, not a parallelogram.

7. Jun 21, 2011

### micromass

Staff Emeritus
Yes, indeed. Sorry for the confusion!

8. Jun 21, 2011

### Staff: Mentor

The figure is symmetrical about that dotted centre line, so concentrate on finding the area of just one of those halves and make it easier for yourself. :surprised

Divide the shape up into simpler shapes whose area you can find, viz., rectangles and triangles. Looks like the height of the lowest triangle will be (X - 30). Add the areas together, and equate them to half of 3375 cm2

You end up with one equation and one unknown. Solve for X.

9. Jun 21, 2011

### Femme_physics

Hmm.. I see what you're saying! That's brilliant.

I can just take a trapezoid area and add the triangle's area, like that

http://img695.imageshack.us/img695/5277/triy.jpg [Broken]

Though from some reason I've still gotten the wrong X. Is this the right equation?

Last edited by a moderator: May 5, 2017
10. Jun 21, 2011

### micromass

Staff Emeritus
Yes, that seems to be the correct formula. Weird that you find the incorrect X now...

11. Jun 21, 2011

### Antiphon

My late Uncle Jor-el says it's our old family crest. He says it should have a big "S" in it.

12. Jun 21, 2011

### eumyang

That's a bit more work than Micromass' method, IMO.

What did you get for X, and what was the answer supposed to be?

13. Jun 21, 2011

### Femme_physics

It's X = 75 which I actually got -- I forgot to multiply the other side by 2 *oops*

Thanks, everyone!