Răspuns :
11 × ( 5²¹ - 5²⁰ - 5¹⁸ ) = 11 x 5¹⁸ ( 5³ - 5² - 1) = 11 x 5 ¹⁸ x ( 125 - 25 - 1) =
= 11 x 5 ¹⁸ x 99 = 11 x 5¹⁸ x 9 x 11 = 11² x 3² x ( 5⁹)² = ( 11 x 3 x 5⁹) ²
11·(5²¹ - 5²⁰ - 5¹⁸) =
11·5¹⁸·(5³- 5²- 1) =
11·5¹⁸·99 =
11·5¹⁸·3²·11 =
11²·3²·5¹⁸=
(11·3·5⁹)² = patrat perfect