Question

Determine a PA onde a5 = 15 e a8 = 21 ?

Answer 1

$a_m=a_n+(m-n)r$

$a_8=a_5+(8-5)r$

$21=15+3r$

$6=3r$

$r=2$

$a_8=a_1+(7)r$

$21=a_1+14$

$a_1=7$

PA de razao=2 e a1=7