moeee

Is there someone who could help me re-arrange this Formula please?

The Formula is

a = 0.15 * b * b + b

I need to calculate "b" in terms of "a" so I can place the Formula into a cell in Excel Program.

verad

Its a long time ago since I did quadratic equations but if my memory serves me right the solution is

b= (-1 +/- (1+0.6a)^1/2)/0.3

intelliGEORGE

a = 0.15b * b + b

a = 0.15b2 + b

a = b + 0.15b2

a + -1b + -0.15b2 = b + -1b + 0.15b2 + -0.15b2

b + -1b = 0

a + -1b + -0.15b2 = 0 + 0.15b2 + -0.15b2

a + -1b + -0.15b2 = 0.15b2 + -0.15b2

0.15b2 + -0.15b2 = 0.00

a + -1b + -0.15b2 = 0.00


Now I'm confused!

moeee

In my Formula , I substituted 1.5 for b , and that gives a = 1.8375

Therefore, if I substitute 1.8375 for a in your Formula Verad, then b should come up as 1.5

0.6 * 1.8375 = 1.1025
1 + 1.1025 = 2.1025
2.1025^1/2 = 1.45
-1 + 1.45 = .45
.45/.3 = 1.5



Thank You very much Verad.

And thank you for your efforts as well IntelliGeorge.

I don't suppose you could explain how you got the Formula Verad, in a similar way as how Intelligeorge tried to show?
Not important as I have what I needed, but am curious, and Giving the Method is much more beneficial to the pupil than giving only the amswer.
If you are busy I completely understand and very much appreciate what you have given regardless.

tytower

so how do you get " +/- " into a spreadsheet? Don't work on mine ? Using two columns for the formula ,one with + and the other with - I get different results, strange ? a with + b a with - (-1 + (1+0.6* B4)^1/2)/0.3 (-1 - (1+0.6* B4)^1/2)/0.3 4.33333 16.00000 -21.00000 8.33333 10.00000 -21.00000 9.33333 11.00000 -21.00000 369.33333 371.00000 -21.00000 -0.16667 1.50000 -21.00000 0.17083 1.83750 -21.00000 -1.66667 0 -21.00000 -1.66667 0 -21.00000

I gave up tryying to format this , its a joke

moeee

Probably won't work on mine either, but thats not what I put in mine.
I just used the PLUS.
It works fine and gives the correct answer.
If instead I use the Minus, the answer comes out wrong.

Verad was going a little on memory and perhaps his memory was almost perfect except for this little bit.
I'm happy...and very grateful.

pak_indo

the formula verad gave is used to solve quadratic equations, ie your equation - of the form

y = ax^2 + bx + c ------or in your case-------- a = 0.15b^2 + b

I have attached a graph of your equation here:



the roots of the equation are where the line crosses the horizontal axis - what you are looking for is the same thing essentially, just shifted up above / below the horizontal axis depending upon your value of a

so, the +/- is supposed to be there, its just that it is not always a useful solution

verad

Quadratic equations have two solutions so the + and - is supposed to be there.
To go through the derivation of the solution is a bit complicated but if you do a google search on solution to quadratic equation the 4th item explains it.