2.1.A.sagews

from sympy import *

d1, d2, l = var('d1 d2 l')
x         = var('x')

# d(x):
dx    = d1 + (d2 - d1) * x/l
# d(x)^2:
dxq   = dx*dx
# A(x):
Ax    = pi/4 * dxq

sol = integrate(1/Ax, (x,0,l))
print latex(simplify(sol))


# result:
# $4 \frac{l}{\pi d_{1} d_{2}}$

\frac{4 l}{\pi d_{1} d_{2}}