# Out of practice, need help for work

• Jan 5th 2011, 01:10 PM
DavidAP
Out of practice, need help for work
Hello, I hope this is in the correct forum. If not, please direct me to the correct place. It's been 20 years since I've had to do any serious math, and I'm quite rusty.

In the equation D/E=(A/B)/(G/H) how do I solve for D when I don't know E, but I know the result of D/E? How would I solve for E when I don't know D, but I know the result of D/E?

Thank you.
• Jan 5th 2011, 01:16 PM
dwsmith
$\displaystyle d=\frac{a}{b}*\frac{h}{g}*e$

$\displaystyle e=\frac{g}{h}*\frac{b}{a}*d$
• Jan 6th 2011, 05:02 AM
DavidAP
Thanks, I think I tried that originally, but when those formulas are plugged into a spreadsheet, a circular reference is created and the formulas fail. How can we solve for D when we don't know E?

A little background: the original equation comes from the TV world, where DAR=PAR*(W/H). That's Display Aspect Ratio equals Pixel Aspect Ratio times (Width divided by Height). From that equation I came up with A/B=D/E*(F/G). From there I came up with D/E=(A/B)/(F/G).

A common value for DAR is 4/3, and common values for Width and Height are 720 and 480. From there it's easy to figure out that PAR=0.888889. What I need is to know are the two numbers that when the second is divided into the first (D/E) the quotient is 0.888889. From reading reference materials and trial and error with spreadsheet formulas, I know that if F/G=720/480 and A/B=4/3, then D/E=8/9, and if F/G=720x480 and A/B=16/9, then D/E=32/27, but I don't know how to arrive at those numbers on my own. For example, what's D if A/B=16/9 and F/G=720/352? What's E?

Dave
• Jan 6th 2011, 06:30 AM
rebghb
Quote:

Originally Posted by DavidAP
Thanks, I think I tried that originally, but when those formulas are plugged into a spreadsheet, a circular reference is created and the formulas fail. How can we solve for D when we don't know E?

A little background: the original equation comes from the TV world, where DAR=PAR*(W/H). That's Display Aspect Ratio equals Pixel Aspect Ratio times (Width divided by Height). From that equation I came up with A/B=D/E*(F/G). From there I came up with D/E=(A/B)/(F/G).

A common value for DAR is 4/3, and common values for Width and Height are 720 and 480. From there it's easy to figure out that PAR=0.888889. What I need is to know are the two numbers that when the second is divided into the first (D/E) the quotient is 0.888889. From reading reference materials and trial and error with spreadsheet formulas, I know that if F/G=720/480 and A/B=4/3, then D/E=8/9, and if F/G=720x480 and A/B=16/9, then D/E=32/27, but I don't know how to arrive at those numbers on my own. For example, what's D if A/B=16/9 and F/G=720/352? What's E?

Dave

:)

Yeah, you don't...
You only do know the ratio.
Like saying, to every 5 men in the place I am there are 6 women.
Now, does this mean that the place has 5 men and 6 women? What about 500 men and 600 women? The correct answer cannot be specified here...
Now in your problem, if you have another equation say $D \times E =$ something or simply $D$ or $E$ equal something, then you can get the other :D
• Jan 6th 2011, 08:43 AM
DavidAP
Trial and error it is, then.

Thanks!
• Jan 6th 2011, 11:19 PM
CaptainBlack
Quote:

Originally Posted by DavidAP
Thanks, I think I tried that originally, but when those formulas are plugged into a spreadsheet, a circular reference is created and the formulas fail. How can we solve for D when we don't know E?

A little background: the original equation comes from the TV world, where DAR=PAR*(W/H). That's Display Aspect Ratio equals Pixel Aspect Ratio times (Width divided by Height). From that equation I came up with A/B=D/E*(F/G). From there I came up with D/E=(A/B)/(F/G).

A common value for DAR is 4/3, and common values for Width and Height are 720 and 480. From there it's easy to figure out that PAR=0.888889. What I need is to know are the two numbers that when the second is divided into the first (D/E) the quotient is 0.888889. From reading reference materials and trial and error with spreadsheet formulas, I know that if F/G=720/480 and A/B=4/3, then D/E=8/9, and if F/G=720x480 and A/B=16/9, then D/E=32/27, but I don't know how to arrive at those numbers on my own. For example, what's D if A/B=16/9 and F/G=720/352? What's E?

Dave

Assume A,B,F,G are all integers, if

D/E=(A/B)/(F/G)

then:

D/E=(A/B)(G/F)=AB/GF

Now reduce the right hand side to lowest terms by dividing top and botton by their greatest common divisor:

D/E=(AB/gcd(AB,GF))/(GF/gcd(AB,GF))

But note this is only the ratio of D to E as a ratio of integers in lowest terms, it does not tell you what D and E are.

CB