Question

Calcule a1 na P.A em que a6=17 e a11=20

Answer 1

$a_6 = a_1 + 5r\\a_1+5r=17\\\\a_{11} = a_1 + 10r\\a_1+10r=20\\\\\\-a_1-5r=-17\\a_1+10r=20\\\\5r=3\\r=\frac{3}{5}\\\\a_1+5r=17\\a_1+5\cdot\frac{3}{5}=17\\a_1+3=17\\a_1=17-3\\\boxed{a_1=14}$

Answer 2

resolução!

a11 = a6 + 5r

20 = 17 + 5r

20 - 17 = 5r

3 = 5r

r = 3/5

a6 = a1 + 5r

17 = a1 + 5 * 3/5

17 = a1 + 15/5

a1 = 17 - 15/5

a1 = 70/5

a1 = 14