# Is this right?

• Jul 10th 2013, 09:23 PM
keosan22
Is this right?
Kern & Bland solid mensuration page 20 number 6

Find the volume and total area of the largest cube of wood that can be cut from a log of circular cross section whose radius is 12.7 in.

I know 12.7 in is the radius and is also half the diagonal of the square (25.4 in)

I also know d = a sqrt2 and since I know d is 25.4 I can calculate a as 25.4 divided by sqrt 2

which is 17.9605 in.

I know Volume is a3
and total area is 6a2

and the answer on the book is in ft. so I'd have to divide by 12 so I'd get 1.4957 ft.

answer should be 3.3528 cu. ft and 13.441 sq. ft.

is this right?
• Jul 11th 2013, 03:06 AM
chiro
Re: Is this right?
Hey keosan22.

Just out of curiosity, how did you derive d = a * sqrt(2)? Were you given that formula?
• Jul 11th 2013, 05:43 AM
Prove It
Re: Is this right?
Quote:

Originally Posted by chiro
Hey keosan22.

Just out of curiosity, how did you derive d = a * sqrt(2)? Were you given that formula?

Any square has a diagonal that is \displaystyle \displaystyle \begin{align*} \sqrt{2} \end{align*} times as long as its side length. Draw a square and use Pythagoras to check.
• Jul 11th 2013, 04:40 PM
chiro
Re: Is this right?
It's a good idea to ask these kinds of questions to students especially if they are having troubles. I do understand the derivation myself.