  The half-infinite XXZ spin chain with a triangular boundary is considered in
the massive regime. Two integral representations of correlation functions are
proposed using bosonization. Sufficient conditions such that the expressions
for triangular boundary conditions coincide with those for diagonal boundary
conditions are identified. As an application, summation formulae of the
boundary expectation values $\langle \sigma_1^a\rangle $ with $a=z,\pm$ are
obtained. Exploiting the spin-reversal property, relations between $n$-fold
integrals of elliptic theta functions are extracted.
