Observam prin calcul direct ca : ca 5 /( 1 * 6 ) = 1 / 1 - 1 / 6 ;
5 / ( 6 * 11 ) = 1 / 6 - 1 / 11 ;
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5 / ( 201 * 206 ) = 1 / 201 - 1 / 206 ;
Atunci suma A = 1 / 1 - 1 / 6 + 1 / 6 - 1 / 11 + ......... + 1 / 201 - 1 / 206 =>
A = 1 / 1 - 1 / 206 => A = 205 / 206 ;
Bafta
