Question

5°) Em um PA dabe-se a3=15 e a6 = 30, então a2 + a4 é:

20
25
30
35
40​

Answer 1

$a_3=15, \ a_6=30, \ a_1=? \\\\a_6=a_1+(n-1).r\Rightarrow 30=a1+5r\\a_3=a_1+(n-1).r\Rightarrow 15=a1+2r\\\\15=3r \Rightarrow r=5\\\\15=a1+2.5\Rightarrow a1=5$

$a_2=?, \ a_4=?\\\\a_2=a_1+(2-1)r\Rightarrow a_2=5+5=10\\a_4=a_1+(4-1)r\Rightarrow a_4=5+15=20\\\\a_1+a_4=10+20\Rightarrow a_1+a_4=30$