Question

Cum il aflam pe a(unu) ?....deci a20=55; r=3.Calculati a(unu)??

Answer 1

$a_{20}=55,r=3,a_1=? \\ a_{20}=55 \Rightarrow a_{20-1}+r=55 \Rightarrow a_{19}+r=55 \Rightarrow a_1+19r=55 \Rightarrow \\ \Rightarrow a_1+19 \cdot 3=55 \Rightarrow a_1+57=55 \Rightarrow a_1=55-57 \Rightarrow a_1=-2$