Drawing tangent line on a curve

[Meeraan]

Hi All !

Can we draw tangent to a curve in excel graphs ....?
If yes if i could have some demo....** this...
I am working on finding rate ** change at an instantaneous point and average rate ** change ie: secant line

Thanks in advance...

[MrShorty]

Short but useless answer, Yes, we can draw tangent and secants for a given curve in Excel. However, Excel is not going to do it for us.

The details ** how to do it will depend on how the curve is defined. If I assume we are working from an equation (a polynomial y=a+bx+cx^2+dx^3+.... for example), then the general approach will look something like:

1) Select the point x0 where want to find the tangent line. Calculate y0=f(x0) at this point
2) From your calculus studies, take the derivative dy/dx ** the function, then use that to calculate the slope m ** the tangent line.
3) From x0,y0 and m, you should be able to calculate x,y values along the tangent line in a new column in Excel. Add this column/data series to the chart.

Plotting a secant line will be similar.

1) Select the two x values (x1 and x2) for the secant line. Calculate the corresponding y values (y1=f(x1), y2=f(x2)).
2) If you store these two points in a convenient place in the spreadsheet, you can then use the TREND() function to calculate other y values along the secant line in a new column.
3) Add this column to the chart.