Question

Determinar o primeiro termo de uma p.a em que o vigesimo termo è 99 e a razao è igual a 5

Answer 1

$a_n=a_1+(n-1).r \\ 99=a_1+(20-1).5 \\ 99=a_1+19.5 \\ 99=a_1+95 \\ 99-95=a_1 \\ a_1=4$

Sendo assim:

(4, 9, 14, 19, 24, 29, 34, 39, 44, 49, 54, 59, 64, 69, 74, 79, 84, 89, 94, 99)

Answer 2

$a_{n} = a_{20} =99\\ a_{1} =?\\n=20\\r=5\\\\\\ a_{n} = a_{1} +(n-1)*r\to \\\\\\99 = a_{1} +(20-1)*5\to \\\\ 99 = a_{1} +(19)*5\to\\\\ 99 = a_{1} +95\to \\\\99-95 = a_{1} \to\\\\ a_{1}=4$