Lets look at available choices: 1) [tex]a(n)=10(2)^n[/tex] [tex]a(1)=10*2=20[/tex] wrong as a(1) should be 10 2) [tex]a(n)=10*2^{n-1}[/tex] is the right choice because if you put 1 as n in the first case you will get: [tex]a(1)=10*2^{1-1}=10*2^{0}=10*1[/tex] 3) [tex]a(n)=10+2n[/tex] [tex]a(1)=10+2=12[/tex] wrong as a(1) should be 10 4) [tex]a(n)=10+2(n-1)[/tex] [tex]a(1)=10[/tex] right but... [tex]a(2)=10+2=12[/tex] wrong as a(2) should be 20
3) and 4) can be dismissed, at once, because the is addition sign between them, and consecutive numbers should be multiplied.
