Music theory is muscular; it requires mental exercise. If a student copies the answer key without analyzing why the answer is correct, they fail to develop the "theory ear." This becomes painfully obvious during exams, where answer keys are not available, or during sight-singing and ear-training labs where the student cannot predict how a chord should sound.
For decades, Bruce Benward and Marilyn Saker’s Music in Theory and Practice, Volume 1 has stood as a cornerstone of undergraduate music education. Now in its 8th edition, this text remains the gold standard for introducing students to the fundamental building blocks of Western music—from basic notation and intervals to diatonic chords, voice leading, and basic harmonic progression. music in theory and practice volume 1 8th edition answer key
The academic journal Music Theory Online often publishes model answers for common textbook exercises. Search their archives. Music theory is muscular; it requires mental exercise