Extrapolation and Interpolation questions

W

wenlianglo

Hi there,

I am currently facing few problems with my report. The first thing is
how do I extrapolate a graph? I have a series of data from experimen
which shows roughly a "mountain" like graph where there is ascendin
zone, peak zone, then descending zone. However, I didn't have enoug
value in the descending zone to plot a longer graph. How do
extrapolate?

Second question, is how to find X value. Imagine the "mountain" shape
graph. At one Y axis, there should be 2 X axis value since there a
ascending and descending zone in the graph. Is there any function t
help me find the 2 X value assuming I know what Y value to look at?

I hope to add more details which hopefully does not confuse anyone
Assuming my graph have 15 X and Y value which made 15 points. Say, firs
10 points is from ascending to peak zone of the graph while point 11
15 is in the descending zone. If I assumed my Y value to read the 2
axis value is one of the first 10 points, say point 7, drawing
horizontal line to the descending zone, it's between point 12 and poin
13. So, by theory, I should interpolate point 12 and point 13 to get th
second X value from the Y axis starting at point 7. How do I write a
equation to let excel do this automatically?