a21=?
an=n+an+2
a1=? cvp:-98
https://img802.imageshack.us/img802/...at11042013.pngcvp:15
https://img845.imageshack.us/img845/...at11042013.pngcvp:64
https://img442.imageshack.us/img442/...at11042013.png
cvp:144
https://img197.imageshack.us/img197/...t11042013h.jpg
cvp:18