Asked by Gavin Seddon
on 13 Feb 2019

Hello

I have plotted a changing distance (d(x)) with time using cftool and calculated the function. Its derivative was calculated and this is v(x). when plotted with fplot the initial Y values are 0. I am calculating the x position at which this rises above the x axes (y>0). To do this i made v(x) equivalent to 1 (==1) and evluated x yet:

>>v(x)==1

ans =

(4119*x^2)/10000 - (1787*x)/5000 + 891/2500 ==1

>> eval x

x =

x

Will someone please help to calculate this? i can measure the x value directly. It is preferable to standardise this procedure.

GS.

Answer by Walter Roberson
on 14 Feb 2019

Accepted Answer

solve(v(x)==1,x)

Gavin Seddon
on 13 Mar 2019

Hello,

is it possible to apply my derivative function (v(x)) to the second column of my table, to produce a third column which will verify my suppersition ? This will also make matters clearer, please?

GS.

Walter Roberson
on 13 Mar 2019

Do you mean that the second column of your table contains x values ? If so then

YourTable.NewThirdColumn = double( subs( v, x, YourTable.{:,2}));

Gavin Seddon
on 14 Mar 2019

Hello,

mytable.NewThirdColumn = double( subs( v, x, MyTable.(2)));

was the command used and yes indeed it produced a third column.

The numbers revealed indicate the method I tried previously was indeterminate. Thank you

GS.

Sign in to comment.

Opportunities for recent engineering grads.

Apply Today
## 2 Comments

## Gavin Seddon (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/444731-calculating-x-when-y-is-0#comment_671201

## madhan ravi (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/444731-calculating-x-when-y-is-0#comment_671202

Sign in to comment.