Resposta:

[tex]\Large \textsf{Leia abaixo}}[/tex]

Explicação passo a passo:

[tex]\Large \boxed{\text{$ \sf a_n = a_1 + (n - 1)\:.\:r$}}[/tex]

[tex]\Large \text{$ \sf 40 = a_1 + (20 - 1)\:.\:2$}[/tex]

[tex]\Large \text{$ \sf 40 = a_1 + 19\:.\:2$}[/tex]

[tex]\Large \text{$ \sf 40 = a_1 + 38$}[/tex]

[tex]\Large \text{$ \sf a_1 = 40 - 38$}[/tex]

[tex]\Large \boxed{\boxed{\text{$ \sf a_1 = 2$}}}[/tex]

[tex]\Large \boxed{\text{$ \sf a_n = a_1 + (n - 1)\:.\:r$}}[/tex]

[tex]\Large \text{$ \sf 43 = 3 + (n - 1)\:.\:5$}[/tex]

[tex]\Large \text{$ \sf 43 = 3 + 5n - 5$}[/tex]

[tex]\Large \text{$ \sf 5n = 43 + 2$}[/tex]

[tex]\Large \text{$ \sf 5n = 45$}[/tex]

[tex]\Large \boxed{\boxed{\text{$ \sf n = 9$}}}[/tex]