Equality involving binomial coefficient in a integral

42 Views Asked by At

Considering the integral I do not visualize and understand why: $$F= \int \rho(\theta_1)d\theta_1 \dots\rho(\theta_n)d\theta_n \sum_{j=1}^n\sum_{i=1}^j \ln(1+e^{-(\theta_i+\theta_j)})\stackrel{?}{=} - \binom{n}{2} \iint d\theta_1 d\theta_2 \rho(\theta_1)\rho(\theta_2) \ln(1+e^{-(\theta_1+\theta_2)}) $$