If you have the Δx, or as you put it, the "step" you don't need "n" to calculate the area using the trapezoidal rule.
in short, Δx=(b-a)/n.
The trapezoidal rule is as follows
Δx[½f(x0)+f(x1)+f(x2)+...+f(xn-1)+½f(xn)] ; where 0, 1, 2, n-1, n are subscripts of x (I just don't know how to use...