Question

afla suma termenilor : zece , unsprezece , doisprezece ai sirului : 9 x 1 , 14 , 9 x 3 , 32 , 9 x 5 , ...

Answer 1

a1=9*1
a2=9*1+5
a3=9*3
a4=9*3+5
a5=9*5
a6=9*5+5


9*1, 9*1+5, 9*3, 9*3+5, 9*5, urmeaza 9*5+5
deci a2p+1= 9*(2p+1)
si a2p= 9(2p-1)+5


atunci a10=9*9+5
a11=9*11
a12=9*11+5
 a10+a11+a12= 9(9+11+11)+5+5=9*20+99+10=180+10+99=190+99=289