If m∠A = m∠B and m∠A + m∠C = m∠D, thenm∠B + m∠C = m∠D.

Since m∠A = m∠B, we can replace m∠A with m∠B in the equation m∠A + m∠C = m∠D. This gives m∠B + m∠C = m∠D, so the statement uses the substitution property.

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