Q: Verify the projection formula for a 3-4-5 triangle (right-angled at C). Angles: A opposite 3, B opposite 4, C = 90°. Check a = 3: b·cosC + c·cosB = 4·0 + 5·cosB. cosB = adjacent/hyp = 3/5. So 5×(3/5) = 3. ✓