Simple Math Problem

[Netopia]

I wanted to make a little formula in a spread sheet to figure the height and width of widescreen flat panels.

Assuming a 16:9 ratio

Assuming that A^2 + B^2 = C^2

And an assumption on my part that I could add the 9 and the 16 to get 25... and then figured that the two sides would be 9/25 and 16/25 of the total length of the two sides combined (this might be where I went wrong)

I wrote the following:

A= squrt ( C^2 * 9/25 )

B= squrt ( C^2 * 16/25 )

I tried it with a triangle with the measurements of

A= 3
B= 4
C =5

A = squrt ( 5^2 * 9/25 )
A = squrt ( 25 * 9/25 )
A = squrt ( 9 )
A = 3


B = squrt ( 5^2 * 16/25 )
B = squrt ( 25 * 16/25 )
B = squrt ( 16 )
B = 4

So that seemed to work ok, but when I try it with real monitor measurements (like the 15.4 inch display on my laptop) I get crap.

I'm NOT a educated math person... I got stoned most of the way through High School Algebra I (in the 70's) and never bothered with anything beyond that (save Geometry). I'm not bad with math, just not versed in the language of it.

Where am I going wrong here?

Joe

 

[CycloWizard]

You have two pieces of information:
1. A^2+B^2=C^2 (Pythagorean theorem)
2. B/A=16/9 (known aspect ratio)

Solve 2 for B (B=16*A/9) and substitute this result into 1. This gives A=C/sqrt(1+256/91). Make a column for your values of C, then another with values of A using this equation. Then make another for B that is simply 16/9 times the result of the column of A.

Which brings me to my next point: don't smoke crack.

 

[cmbehan]

Originally posted by: CycloWizard
You have two pieces of information:
1. A^2+B^2=C^2 (Pythagorean theorem)
2. B/A=16/9 (known aspect ratio)

Solve 2 for B (B=16*A/9) and substitute this result into 1. This gives A=C/sqrt(1+256/91). Make a column for your values of C, then another with values of A using this equation. Then make another for B that is simply 16/9 times the result of the column of A.

Which brings me to my next point: don't smoke crack.

yup. This is how I would do it:

A=(B*16)/9
B=(A*9)/16
C=sqrt(a^2+b^2)

That means that:
C=sqrt(A^2+((A*9)/16)^2)
and
C=sqrt(((B*16)/9)^2+B^2)

 

[Netopia]

Thanks guys... I'll have to plug it in, look at it... and try to soak it in down to the understanding level!

Joe

 

[Aluvus]

http://tvcalculator.com/

CycloWizard's method looks good to me.