HELP Which represents a recursion function?

A. Xn = f(Xn)

B. Xn = f(X0)

C. Xn = f(a1)

D. Xn = f(Xn-1)

(Picture added for more info)

Choice D

This basically says "the nth term is found by computing math, aka evaluating some function, based on the (n-1)th term".

As an example, we could have a function that simply squares the input

Let's say we started at 5. It squares to 25 which in turns squares to 625, and so on. Each new term is based on the old term. Namely, we squared the old n-1 term to get the new nth term.

the formula for the equation of a circle is (x – h)2+ (y – k)2 = r2, where (h, k) represents the coordinates of the center of the circle, and r represents the radius of the circle. if a circle is tangent to the x-axis at (3,0), this means it touches the x-axis at that point.