Răspunsuri

2014-06-10T20:22:43+03:00
Ti-am atasat un fisier cu problema rezolvata!
2014-06-11T02:50:32+03:00
Este o progresie aritmetica, si avem:
 a_{1}=1 \\  \\   a_{n}=31 \\  \\ r=3

S_{11} = \frac{ (a_{1}+ a_{n})*n }{2} \\ \\S_{11}= \frac{(1+31)*11}{2} \\ \\ S_{11}=\frac{32*11}{2} \\ \\ S_{11}=\frac{352}{2} \\ \\ S_{11}=176 \\ \\ a_{n}= a_{1}+r(n-1) \\ \\31=1+3(n-1) \\ \\ 31=1+3n-3 \\ \\ 31=3n-2 \\ \\ 3n=31+2 \\ \\ 3n=33 \\ \\ n=11[/tex]
5 3 5