a=3+3^3+3^5+...+3^2019 ; sunt (2019+1):2=1010 termeni, daca ii grupam cate 3=> 336 de grupe (de cate 3 termeni) si inca 2 termeni.
a=3+3^3+3^5(1+3^2+3^4)+3^11(1+3^2+3^4)+……+3^2015(1+3^2+3^4)
1+3^2+3^4=91
a=30+91(3^5+3^11+….+3^2015)
91(3^5+3^11+….+3^2015) e divizibil cu 91
a:91=[30+91(3^5+3^11+….+3^2015)]:91=3^5+3^11+….+3^2015 rest 30
