Question

( an)-арифметическая прогрессия,a2=4 a12=16,Найдите а7

Answer 1

a(n) - ap пр

a(2) = 4

a(12) = 16

a(7) -?

cистема:

{a(2) = a(1) +d              {4 = a(1) + d        <=> {a(1) = 4-d              {a(1) =2.8

{a(12) = a(1) + 11d  <=> {16 = a(1) + 11d           {16 = 4-d+11d  <=> { d = 1.2

a(7) = a(1) + 6d

a(7) = 2.8 + 6 * 1.2

a(7) = 2.8 + 7.2

a(7)= 10