Trapezoidal Rule

  In either Fortran or C, write a function subroutine of the form

C:        float trapint(float f[],float r[],float dr[],int N)

Fortran:  real function trapint(f,r,dr,N)
          integer N
          real f(N),r(N),dr(N)

which returns the result of using the trapezoidal rule to perform the integral

from values at the N+1 points .

As input to this routine, the user supplies the values , , and in the arrays r, dr, and f, respectively.