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31 ² = ( 30 + 1 )² = 30² + 2 *30*1 + 1 ² = 961
41 ² = ( 40 + 1 )² = 40² + 80 + 1 = 1 681
39 ² = ( 40 - 1 )² = 40² - 80 + 1 = 1 521
57 ² = ( 60 - 3 )² = 60² - 360 + 9 = 3 249
82 ² = ( 80 + 2 )² = 80² + 320 + 4 = 6 724
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31 x 29 = ( 30 + 1 ) ( 30 - 1 ) = 30² - 1 ² = 900 - 1 = 899
49 x 51 = ( 50 - 1 ) ( 50 + 1 ) = 50² - 1 ² = 2 499
103 x 97 = ( 100 + 3 ) ( 100 - 3 ) = 100² - 3 ² = 9 991
