Răspuns:
[tex]9 \times (5 + 10 + 15 + ... + 200) \div 41 = \\ \\ = 9 \times( 5 \times (1 + 2 + 3 + ...40) )\div 41 = \\ \\ = 9 \times (5 \times \frac{40 \times 41}{2} ) \div 41 = \\ \\= 9 \times (5 \times 820 )\div 41 = \\ \\ = 9 \times 4100 \div 41 = \\ \\ = 366900 \div 41 = \\ \\ = 900[/tex]
900=30² = pătrat perfect
