Volume= 18 cm3
Height of the pyramid= 3 cm
B1= 2 cm
B2= 3 cm
What is the height of the base?

- anonymous

Volume= 18 cm3
Height of the pyramid= 3 cm
B1= 2 cm
B2= 3 cm
What is the height of the base?

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- anonymous

you mean area of the base?

- anonymous

wait, no. base area is .5xh(b1+b2) so i need the height

- anonymous

i though height was given as 3 cm?

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- anonymous

thats the height of the pyramid

- anonymous

okay, and what do you need to evaluate?

- anonymous

the height of the base

- anonymous

http://en.wikipedia.org/wiki/Pyramid_(geometry)
are you talking about that?
how is there a height of the base?

- anonymous

x=18cm3

##### 1 Attachment

- anonymous

ah, so you should say its a trapezoidal pyramid.

- anonymous

yes

- anonymous

okay, do you know the formula to the volume of a pyramid?

- anonymous

yes. 1/3 x B x H

- anonymous

what is B in the formula?

- anonymous

i dont know. b1 = 2 cm b2= 3 cm

- anonymous

yess.

- anonymous

okay you must always know what the letters mean in a formula.
for example the area of a rectangle is the its length multiplied by its width. you shouldn't remember it as l x b, because when I ask you what l and b are, you don't have an answer to that.

- anonymous

volume of a pyramid is one-thirds the product of the base area and its height

- anonymous

yea

- anonymous

so B is the area of the base.
right,
so substitute H and V in the equation

- anonymous

right

- anonymous

soooo,,,,,,,,,,,,,,,,,,,,

- anonymous

so, tell me what you got after substituting the values of V and H.

- anonymous

V= 18cm H= 3cm. h= i dont know

- anonymous

substitute V = 18, H =3 in
V = 1/3 x B x H and find the value of B.

- anonymous

but H is the height of the pyramid, i need the height of the base (h)

- anonymous

Yes, volume of a pyramid is 1/3 x Base area x Height of the pyramid.
so once you find the base area of the pyramid, substitute its value as 1/2(b1+b2) x h and find h

- anonymous

so how do you find h?

- anonymous

I just told you.
can you please just for the love of god substitute
V and H in
V = 1/3 B x H and give me an answer for B?

- anonymous

BUT I DONT KNOW THE BASE. sorry i'm not good at math, my lord.

- anonymous

what is so hard about this?
18 = 1/3 x B x 3
B =18
but we know that B = 1/2(b1+b2) x h
so 18 = 1/2(2+3) x h
h = 36/5

- anonymous

you just have to work backward to get the base. you won't always find equations in the correct order. you must know how to manipulate equations to get your variable.

- anonymous

where did we get 36

- anonymous

multiply 18 by 2 and divide by 5.

- anonymous

i got h = 7.2. is this correct?

- anonymous

yeah. oooo you used a calculator to find the value of 36/5! good on you!

- anonymous

btw, which grade are you in?

- anonymous

7th

- anonymous

okay. Have you ever done manipulation of equations?

- anonymous

no

- anonymous

like if x = y+2, what is y?

- anonymous

x-2?

- anonymous

then why do you say you haven't done any algebraic manipulation before?

- anonymous

btw, h= 2.4. 1/2 x 2.4 (3+2) = 6. 6x3=18.

- anonymous

why do you say 6 x 3 = 18?

- anonymous

is the formula for volume of a pyramid area of base x height?

- anonymous

because the height of the pyramid is 3 cm, so to find the volume you have to multiply the base by the height.

- anonymous

oh yeah, divide by 3. nevermind...

- anonymous

good, at least you figured that out on your own.

- anonymous

oh stopp

- anonymous

:D

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