# Thread: This is not rocket science but I am stuck!

1. ## This is not rocket science but I am stuck!

Hi,

Well this is the formula:

x(y+500)/(x+y+500) = 1520

x should be near 2249 and y, near 4189.

Could you help me to solve that equation, step by step. My knowledge of algebra is very low and quite far away. ....45 years away!

Thanks!

Alain

2. Originally Posted by AlainB
Hi,

Well this is the formula:

x(y+500)/(x+y+500) = 1520

x should be near 2249 and y, near 4189.

Could you help me to solve that equation, step by step. My knowledge of algebra is very low and quite far away. ....45 years away!

Thanks!

Alain
Hi Alain, is your equation $\frac{x(y+500)}{(x+y+500)} = 1520$?

I'm not exactly sure what it is your trying to do, could you explain further?

3. You cannot solve an equation in two variables for specific values of both variables.

4. Just to add to HallsofIvy's post, if you have $n$ number of variables, you need $n$ number of equations.

With yours above you can get $y$ in terms of $x$, is that what you need?

5. Hi,

Thanks! You see, I would like to use excell to compute the values of 2 resistors in parallel combine with a variable resistor to obtain a certain output resistance value. Like in the image the output value (R eqv.) would be between 1520 and 1660 Ohms when the variable resistor is turned from 0 to 100%. The values computed for the 2 resistors are 2249 and 4189.

It's fairly straightforward, if the resistances are x and y then

x(y+500)/(x+y+500) = 1520
y(x+500)/(x+y+500) = 1660

I was lazy and solved them iteratively in Excel, if you did it manually - you'd get a quadratic, the solutions are:

x=(377000√17+1861000)/(325√17+179)
y=(1625√17+5875)/3

Now if possible I would like to have the step by step sequence for solving that problem. I would then be able to do an Excell sheet to compute different scenario.

Thanks again!

Alain

6. Hi,

Thanks! You see, I would like to use excell to compute the values of 2 resistors in parallel combine with a variable resistor to obtain a certain output resistance value. Like in the image the output value (R eqv.) would be between 1520 and 1660 Ohms when the variable resistor is turned from 0 to 100%. The values computed for the 2 resistors are 2249 and 4189.

It's fairly straightforward, if the resistances are x and y then

x(y+500)/(x+y+500) = 1520
y(x+500)/(x+y+500) = 1660

I was lazy and solved them iteratively in Excel, if you did it manually - you'd get a quadratic, the solutions are:

x=(377000√17+1861000)/(325√17+179)
y=(1625√17+5875)/3

Now if possible I would like to have the step by step sequence for solving that problem. I would then be able to do an Excell sheet to compute different scenario.

Thanks again!

Alain

7. Originally Posted by AlainB
Well, let's keep it simple!

The previous post explain wy I need the equation solved. But we can forget about electronic.

Is it possibe to solve x and y with those 2 equations? Mr F says: Yes. Using a TI-89 I got x = 2248 and y = 4192 (rounded to the nearesr whole number).

x(y+500)/(x+y+500) = 1520
y(x+500)/(x+y+500) = 1660

As previously said the values should be near 2249 and 4189.

Ultimately I would like to have an Excel sheet that will compute different scenarios as needed:

Inputs:
VR...............................500
From R eqv low.............1520
To R eqv high...............1620

Results:
R1..............................2249
R2..............................4189

The Excel part I could probably do by myself if I can just get the final algebra equations.

Thanks!

Alain
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