Question

calcule o a180 da p.a. que possui a20 =55 e r =8

Answer 1

P.A.(
a 180 = ?
a 20 = a 1 + (n - 1 ).r
a 20 = a 1 + (20 - 1 ).8
a 20 = a 1 + 19.8
a 20 = a 1 + 152
a 1 +  152 = 55
a 1 = 55  -  152
a 1 = -97

a 180 = a 1 + (n - 1 ).r
a 180 = -97 + (180 - 1 ).8
a 180 = -97 + 179 . 8
a 180 = -97 + 1432
a 180 = 1335

Answer 2

$a_{20}=a_1+19r\\\\a_1+19r=55\\\\a_1+19\cdot8=55\\\\a_1+152=55\\\\\boxed{a_1=-97}$

$a_{180}=a_1+(n-1)\cdot r\\\\a_{180}=-97+(180-1)\cdot8\\\\a_{180}=-97+1432\\\\\boxed{a_{180}=1335}$