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5+10+15+....625=?
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Răspuns :

5 + 10 + 15 + ....+625 =
n = ( 625 - 5 ) : ( 10 - 5) +1 = 
n = 620 : 5 +1 
n = 124 + 1
n = 125
S = [( 625 +5) 
× 125 ] : 2 =
S = ( 630 × 125 ) : 2 =
S = 78750 : 2
S = 39375.

5x(1+2+3...+125)=5x125x126:2=78750:2=39375