Question

Calculati sumele 2+4+6...+18+20

Answer 1

2 + 4 + 6 + ....... + 18 + 20 = 110

= 2 × ( 1 + 2 + 3 + ........... + 9 + 10 ) = → am dat factor comun pe 2

= 2 × 10 termeni × ( 1 + 10 ) / 2 =

= 10 × 11 =

=110

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sau aplic asociativitatea adunarii, fiind mai putini termeni

2 + 4 + 6 + 8 + 10 + 12 + 14 + 16 + 18 + 20 = 110

= ( 2 + 20 ) + ( 4 + 18 ) + ( 6 + 16 ) + ( 8 + 14 ) + ( 10 + 12 ) =

= 22 + 22 + 22 + 22 + 22 =

= 110