Răspuns: d = 0
Explicație pas cu pas:
a = 1+2+3+4+...+9+10
a = 10 × ( 1 + 10 ) : 2 → aplic formula sumei lui Gauss
a = 5 × 11
a = 55
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b = 6 + 7 + .... + 14 + 15
15 - 6 + 1 = 10 termeni are suma
b = 10 × ( 6 + 15 ) : 2
b = 5 × 21
b = 105
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c = 11 + 12 + ..... + 19 + 20
( 20 - 11 ) + 1 = 10 termeni are suma
c = 10 × ( 11 + 20 ) : 2
c = 5 × 31
c = 155
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d = 209 × [ ( c - b ) - ( b - a ) ]
d = 209 × [ ( 155 - 105 ) - ( 105 - 55 ) ]
d = 209 × ( 50 - 50 )
d = 209 × 0
d = 0
