Need LOTS of help with surface areas! :-(

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:shock: ok, this is my first post here, but i'm desperate for help with this.! i'm taking a geometry course this summer to get a jump start on next year. and i've done alright all throughout the course. but now with only 4 days left, i'm in deep trouble.
i know the formulas for finding the surface areas of things (prisms, cylinders, pyramids, cones, ect.) but when i TRY to plug into them, i get completly WRONG answers! :( and i'm at a total loss as to what to do. if someone can help this make more sense to me, i'd owe you BIG TIME!

thanks a bunch
 
You got to post some problems.
Tell us where you are having trouble.
Tell us what you do not understand.
Give some example of your work.
 
ok sorry bout that.

a right rectangular prism with w=7.5 cm, h= 8cm, l=14.5 cm

How do I find the surface area and lateral area of this? (this is a basic one compared to some of the other strangely shaped ones, but i didn't know how to describe them. so hopefully if i get to understand this one, i can apply it to others.)

I know the equation for the Lateral area is L=Ph and the equation for the Surface area is T= Ph+2B . My problem is that I don't understand what to plug in where. I don't know what to consider a base and what to consider a face, so i end up plugging into it all wrong (i guess) because my answers don't come out right.

In theory this shouldn't be diffucult for me, (just plug in and solve!) but it is really mind blowing for some reason!
 
black0belt0babe7 said:
a right rectangular prism with w=7.5 cm, h= 8cm, l=14.5 cm How do I find the surface area and lateral area of this?
Click to expand...

Top or Bottom: Area1 = l*w = 14.5 cm * 7.5 cm = 108.75 cm<sup>2</sup>
Contributes to Surface Area but NOT to Lateral Surface Area

One of One Pair of Sides: Area2 = l*h = 14.5 cm * 8 cm = 116 <sup>2</sup>
Contributes to Surface Area AND to Lateral Surface Area

One of Other Pair of Sides: Area3 = w*h = 7.5 cm * 8 cm = 60 <sup>2</sup>
Contributes to Surface Area AND to Lateral Surface Area

Lateral Surface Area: 2*Area2 + 2*Area3
Total Surface Area: 2*Area1 + 2*Area2 + 2*Area3 = 2*Area1 + Lateral Surface Area

The formulas you presented - I do not know what those are. What is "P"? Anyway, you really don't need wonder formulas. Make your own by examining the object.
 
lol, see where i'm confused about how to use those dang formlas? (P is perimeter) :?

On some of the shapes, i can do what you said and just figure it out on my own looking at it. But some of them are really weird shaped, and he says to use those "formulas" to find them. And with certain ones i just plain and simple can't figure them out.

i'll try to explain what the equations are 'supposed' to mean.

for lateral area L(lateral alrea)=P(perimeter)h(height)
*see, it confuses me because i dont know which numbers and sides of the shape are what!

and for surface area T(total surface area) = Ph + 2B (and B supposedly means something to do with the bases, but i'm not really what! the book doesn't go into great detail, and neither does the professor obviously because it's a 5 week course.)

final exam is this week, and so i'm really stressing about this.
 
Ah, that is fine. I should have thought of that.

Lateral Surface Area = 2*w*h + 2*l*h = h*(2*w + 2*l) = h*Perimeter

Whether one formula is more useful than another is a matter of at least two things:
1) Which formula do you know?
2) For which formula do you have sufficient information?
 
ok, but when i try to do it, i get wrong answers. so i don't know. it's all very frusterating because i have an A in there, and i'm afraid i will bomb this test and kill my grade.

the way i've been trying it is L=Ph, which would be L= 31 (8), L=248

and T= Ph +2B, which would be 248 + 2(60), and that gives me 368

so i got an L of 248 and a T of 368, and that's wrong! it doesn't say the lateral area in the back of the book, but it says that the Surface area is 569.5 cm
 
black0belt0babe7 said:
a right rectangular prism with w=7.5 cm, h= 8cm, l=14.5 cm
I know the equation for the Lateral area is L=Ph and the equation for the Surface area is T= Ph+2B
Click to expand...
P = 2*7.5 cm + 2*14.5 cm = 15 cm + 29 cm = 44 cm

How did you get 31 cm?

L = 44 cm*8 cm = 352 cm<sup>2</sup>

B = 7.5 cm * 14.5 cm = 108.75 cm<sup>2</sup>

How did you get 60 cm<sup>2</sup>?

A = P*h + 2*B = L + 2*B = 352 cm<sup>2</sup> + 2*108.75 cm<sup>2</sup> = 569.50 cm<sup>2</sup>
 
Gee, got a hard time following what ya'll are saying;
BB, you say you have:
a right rectangular prism with w=7.5 cm, h= 8cm, l=14.5 cm

well, that's simply a box, isn't it?
the 2 ends are each 7.5 * 8 = 60; so 60 * 2 = 120
the top and bottom: 7.5 * 14.5 * 2 = 217.5
the sides: 8 * 14.5 * 2 = 232
Add 'em up: 120 + 217.5 + 232 = 569.5 (as you book says)

Now, what d'heck is "a lateral area" related to a poor box?
Is there such an animal, TK?
 
Lateral Surface Area - everything but the top and bottom?
 
hey everyone. sorry it took me so long to get back. i've figured out how to solve these monsters finally!

(oh BTW, in the equations, you have to plug in the perimeter of the base, not the perimeter of the entire figure. might not have made myself specific enough on that.)

~~~w=7.5 l=14.5 h=8~~~

like i said, i finally got this thing figured out! so, here's how it finally ended up working : Lateral area=Ph, so L is 31 x14.5= 449.5 (again, the P is the perimeter of the base, not of the entire figure. so P= 7.5 + 7.5 + 8 + 8, which is 31. ALSO, the h is the height of the entire figure, not of the base.) L= 449.5

now the surface area equation is Total Surface area =Ph + 2B, so T=449.5 + 2(60), which is 569.5! :) (the B stands for the area of the base, so 7.5 x 8 = 60)


YaY! i'm so excited! lol. y'all helped a lot, made me look at the equations differently. does all that i just tried to explain make sense?i hope so! reply to this and tell me what u think, hope to hear from u soon, and again, thanks!
 
Boy oh boy, no matter how you cook it, we're working with
3 pairs of simple rectangles: why did the Guru's need to
complicate things with stuff like Ph et al........
 
I am pleased that I am not the only one who didn't understand what "P" was doing in there at the beginning. Someone likes to do it "MY WAY", I guess. Blech!
 
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