Question

Calculati suma a 25 de nr nat consecutive stiind ca primul si al treilea numar sunt direct proportionale cu 10 si 14.

Answer 1

a₁
a₂=a₁+1
a₃=a₁+2
a₁/10=(a₁+2)/14
14a₁=10a₁+20
4a₁=20
a₁=5
a₁+a₂+....+a₂₅=
=5+6+......+29=5+(5+1)+.....+5+24=5*25+(1+2+...+24)=
=125+24*25/2=125+300=425