( 4 + 8 + .... + 92 + 96 + 100 ) - ( 2 + 6 + 10 + ..... + 94 + 98 ) = 50
= 4 × ( 1 + 2 + ..... + 23 + 24 + 25 ) - 2 × ( 1 + 3 + 5 + ...... + 47 + 49 ) =
= 4 × 25 × ( 1 + 25 ) : 2 - 2 × [ ( 49 - 1 ) : 2 + 1 ] × ( 1 + 49 ) : 2 =
= 2 × 25 × 26 - 25 × 50 =
= 50 × 26 - 25 × 50 =
= 50 × ( 26 - 25 ) =
= 50
