Is "polyarea" the same as "trapz" for calculating area under the curve?

to find the area of enclosed polygon (or any shape), which works best? polyarea or trapz?

[Roger Stafford]

They may not always give you the same answer. To use 'polyarea' properly you would need to have a closed polygon which would require including the two points on the variable of integration axis at the lower and upper limits of integration. The 'trapz' function will count areas above a curve which has descended to negative integrand values as being negative. Also when the upper integration limit lies below the lower limit, this will produce negative values. I suspect that 'polyarea' would produce different results in these cases, but you can easily find out by experimenting.

[J K]

my prof gave us hint, to calculate the area enclosed, you need to take to forward area subtracted by the backward area. And it is under "trapz"

can you write the code for that?

[J K]

my problem is similar to this. http://www.mathworks.com/matlabcentral/newsreader/view_thread/300254

But I don't understand the 'idx' term. the_area = trapz(x(1:idx),y(1:idx))-trapz(x((idx+1):end),y((idx+1):end)

Plz help

Answer 1

[Veronica Maria Rodriguez Betancourtt]

According to ChatGPT, polyarea uses the Shoelace Theorem.