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Val123's avatar

Geometry help please?

Asked by Val123 (12739points) June 23rd, 2010

It’s me again. I’ve learned this stuff in the past, and I know there are relatively simple formula’s for figuring them out, but the online courses seem to go out of their way to make things sound more complicated and important than they are.

What formulas do I use to find the following:

Find the area of a regular pentagon with side equal to 3 and apothem equal to K. (apothem kills me! I feel like I’m trying to say ‘possum’ but can’t!)

Find the area of a regular hexagon with a 48-inch perimeter.

Find the area of a triangle with base of 10 inches and altitude to the base of 16 inches.

Find the area of a parallelogram with sides of 6 and 12 and an angle of 60°.

Find the area of a trapezoid with bases of 8 and 16 and a height of 10.

LAST QUESTION! The student brought this to me because it seemed counter-intuitive. It’s a square so you would assume the angles would be =, but the answer key says it’s >

Choose the relationship symbol to make a true statement.

<
=
>

LE ___ VO

Can’t post the pic, but it was a square with 4” sides. Starting at top left and moving to the right and on around, the angles were labeld E V O L.
Then there was a diagonal line from L (bottom left) to V. At L the line was labeld 45 degrees At V it’s labeld 46 degrees…. wait. I just saw the discrepancy in the degrees. Is the > answer due to the discrepancy in degrees? It’s kind of making sense now. Please help anyway….

THANK YOU!!!

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16 Answers

ragingloli's avatar

Protip: regular N-gons can be divided into identical triangles, trapezoids and parallelograms can all be split in triangles and rectangles. Calculate their areas independently and add them at the end.

Response moderated
ragingloli's avatar

Also, if the angles are different, then it is not a square.

ragingloli's avatar

also, what is ‘K’?

Val123's avatar

@ragingloli That’s exactly my question…if it’s 4…wait…... It notes 4” on the bottom and top lines, but nothing on the side lines. So…it’s not an exact square. I also learned that about splitting them into triangles and rectangles, but not sure how to apply it.
SO….if LE is 46 degrees and VO is 45 degrees, then LE is > VO. Is that right?

Don’t know what you’re talking about with K.

ragingloli's avatar

here is what my book says about the pentagram case:

I will just assume that the side means the side of the base of the triangles.

so you have a = 3
φ = 360/n
A = n x An (n is the number of the triangles in the ngon)
An = ½ x r² x φ

You do not have r, so you have to calculate it:
a = 2r x sin φ/2
you already have a, so…
a / sin φ/2 = 2r
r= (a / sin φ/2) x ½

Therefore:
An = ½ x ((a / sin φ/2) x ½)² x φ
A = (½ x ((a / sin φ/2) x ½)² x φ) x n

no guarantees, though, last geometry class was years ago.

ragingloli's avatar

now for the hexagram

I will assume that perimeter means the sum of all sides of the hexagram and not the circumference of the circle that surrounds the hexagram

so
p = 48
and
p = n x a
therefore
p/n = a
thus
a = 8

now you have a, repeat the same procedure you did for the pentagram (remember to recalculate φ)

ragingloli's avatar

the triangle
A = ½ x g x h (g is the base, h is the altitude I presume)

ragingloli's avatar

parallelogram (I found equations without having to split it into triangles and rectangles)

A = a x b x sin α

Trapezoid

A = ½ x ( a + c ) x h

ragingloli's avatar

So and here is the parallelogram with triangular dissection:
A = 2×½ a x h + h x b (a is the small side of the triangle, b is 12 – a , h is the height of the parallelogram)
A = ah + hb
A = h(a+b)
A = 6sinα x ( 6cosα + 12 – 6cosα )

ragingloli's avatar

For the dissected trapezoid
A = ch + ((a-c)h)½ (with a the longer side, c the short side and h the height)

but as you can see
A = ch + (ah-ch)½
A = ch + ½ah -½ch
A = ½ch + ½ah
A = ½(ch+ah)
A = ½(c+a)h
it is the same as the equation I already gave you above.

Val123's avatar

@ragingloli I gotta “book” too. That’s part of the problem. I need a human to talk to.

ragingloli's avatar

Addition to the dissected parallelogram
A = 6sinα x (6cosα +12 -6cosα )
A = 6sinα x 12
A = sinα x 6×12

Also the same as the one in the book.
Sometimes I make it harder for myself as needed lol.

I gotta “book” too. That’s part of the problem. I need a human to talk to.
Any problems understanding what I wrote?

ragingloli's avatar

also, are the angle labels on the opposite sides of the LV diagonal?

Val123's avatar

That’s just it…they LOOK diagonal. If this isn’t a perfect square, it would take the Hubble telescope to see it….I’ll draw a picture and post it.

No problem understanding what you wrote if I have an hour to puzzle each post out. As it is, I have, maybe, 5 minutes to figure it out before I have to go help another kid with English, or Lit, or Natural Science. I need a micro-wave version.

ragingloli's avatar

I made some pictures for you.
Here
Here
and Here

I am way too nice to you.

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