Question

Sa se determine suma primilor 10 termeni a unei progresie aritmetica stiind ca a3=5 si a6=14

Answer 1

$\left \{ {{a3=a1+2r} \atop {a3=a1+2r}} \right. =\ \textgreater \ \left \{ {{5=a1+2r}|(-1) \atop {14=a1+5r}} \right. =\ \textgreater \ 9=3r =\ \textgreater \ r=3$
5=a1+6 => a1=1
=> a10=a1+9r=1+27=28
S10=[(a1+a10)10]/2=(1+28)/5=29/5