k ( 2 k - 1 ) ( 2 k + 1 ) / 3 + 3 ( 2 k + 1 )² / 3 =
= k ( 4 k² - 1 ) / 3 + 3 ( 4 k ² + 4 k + 1 ) / 3 =
= ( 4 k³ - k ) / 3 + ( 12 k² + 12 k + 3 ) / 3 =
= ( 4 k³ + 12 k ² + 11 k + 3 ) / 3
sau dai factor comun pe 2 k + 1
= ( 2 k + 1 ) ( 2 k² - k + 6 k + 3 ) / 3 =
= ( 2 k + 1 ) ( 2 k² + 5 k + 3 ) / 3
