☰ Contents1.2 · The Fundamental Theorem of Arithmetic35/101

Worked: a hidden common factor

7 × 11 × 13 + 13 = 13 × (7 × 11 + 1) = 13 × 78. The 13 sits in both terms, so it divides the whole thing — composite.

Check yourself

Quick check
Explain why 7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 is composite.