Question

Daca $S_{n} = 5 n^{2} +6n$ aflati r si $a_{1}$

Answer 1

Sn=a1.n+r(n-1)/2
dar Sn=n(5n+6)
n(5n+6)=a1.n+r(n-1)/2
aducem la acelasi numitor si simplificam cu n
10n+12=2a1 +rn-r
10n+12=r.n+2.a1-r
10n=rn
r=10 ratia
2a1-r=12
2a1=12+r=12+10=22
a1=11
deci a1=11
r=10