r d r = (a_{2} + m \frac{λ}{2}) \frac{λ}{2} \approx a_{2} \frac{λ}{2}

${r \mathrm{d}r = \bigg(a_2 + m \frac{\lambda}{2}\bigg) \frac{\lambda}{2} \approx a_2 \frac{\lambda}{2}}$