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6+11+16+21+.......+506
Vă rog !


Răspuns :

6+11+16+21+..+506=
(1+5)+(1+5×2)+(1+5×3)+(1+5×4)+...+(1+5×101)
=1×101+5+5×2+5×3+5×4+...+5×101
=101+5(1+2+3+...+101)
=101+5×101×102:2
=101+25755
=25856