MATLAB Answers

## calculating x when y is > 0

Asked by Gavin Seddon

### Gavin Seddon (view profile)

on 13 Feb 2019
Latest activity Commented on by Gavin Seddon

### Gavin Seddon (view profile)

on 14 Mar 2019
Accepted Answer by Walter Roberson

### Walter Roberson (view profile)

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.

Gavin Seddon

on 14 Feb 2019
Hello, I tried
solve (x)
ans =
0
does this help?
GS.
madhan ravi

### madhan ravi (view profile)

on 14 Feb 2019
Are you asking or stating?

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## 1 Answer ### Walter Roberson (view profile)

Answer by Walter Roberson

on 14 Feb 2019
Accepted Answer

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

Gavin Seddon

### Gavin Seddon (view profile)

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

### Walter Roberson (view profile)

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

### Gavin Seddon (view profile)

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.

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