Răspunsuri

2014-10-19T21:55:02+03:00
S=2+5+8+...+89=
=3-1+6-1+...+90-1=
=(3+6+9+...+90)-30=
=3(1+2+3+...+30)-30=
=3*30*31/2-30=
=1395-30=
=1365

PS sper ca nu am gresit la calcul, in orice caz asta e metoda de rezolvare