Question

81+86+91+96+...+491=

Answer 1

Răspuns:

Explicație pas cu pas:

81 + 86 + 91 + ..... + 491 =

( 491 - 81 ) : 5 + 1 = 410 : 5 + 1 = 83 ->  termeni are suma ( cu ratia=5)

= 83 × ( 81 + 491 ) : 2 =

= 83 × 572 : 2 =

= 83 × 286 =

= 23 738

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Sau:

( 1 + 6 + 11 + ..... + 491 ) - ( 1 + 6 + 11 + 16 + ..... + 76) =

( 491 - 1 ) : 5 + 1 = 490 : 5 + 1 = 99 termeni are prima suma

( 76 - 1 ) : 5 + 1 = 75 : 5 + 1 = 16 termeni are a doua suma

= 99 × ( 1 + 491) : 2 - [ 16 × ( 1 + 76 ) : 2 ) =

= 99 × 492 : 2 - ( 8 × 77 ) =

= 99 × 246 - 616 =

= 24 354 - 616 =

= 23 738

Answer 2

81+86+91+...+491=?

(491-81):5+1=410:5+1=83

=83x(81+491):2=

=83x572:2=

=83x286

=23 738