Răspuns:
Explicație pas cu pas:
f)
25 + 50 + 75 + 100 + ..... + 400 =
= 25 × ( 1 + 2 + 3 + 4 + ...... + 16 ) =
= 25 × [ 16 × ( 1 + 16 ) : 2 ] =
= 25 × ( 8 × 17 ) =
= 25 × 136 =
= 3 400
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g)
10 + 20 + 30 + 40 + ...... + 480 =
= 10 × ( 1 + 2 + 3 + 4 + ..... + 40 ) =
= 10 × [ 40 × ( 1 + 40 ) : 2 ] =
= 10 × ( 20 × 41 ) =
= 10 × 820 =
= 8 200
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h)
6 + 12 + 18 + 24 + ...... + 1 200 =
= 6 × ( 1 + 2 + 3 + 4 + ...... + 200 ) =
= 6 × [ 200 × ( 1 + 200 ) : 2 ] =
= 6 × ( 100 × 201 ) =
= 6 × 20 100 =
= 120 600