Question

determine o primeiro termo de uma p.a de razao 3 e a20: 30.

Answer 1

An =A1 + (n-1)r
A20 = 30
n = 20
r = 3
30 = A1 + (20-1)•3
30 = A1 + 57
A1 = -27

Answer 2

$\mathsf{a_1=a_n-(n-1)\cdot r }$

$\mathsf{ a_1=30-(20-1)\cdot3}$

$\mathsf{a_1=30-19\cdot3 }$

$\mathsf{ a_1=30- 57}$

$\boxed{\boxed{\mathsf{ a_1= -27}}}$