tính a = 1-3+3^2-3^3+3^4-…+3^98-3^99+3^100

2 bình luận về “tính a = 1-3+3^2-3^3+3^4-…+3^98-3^99+3^100”

1. Có :

   a = 1-3+3^2-3^3+3^4-…+3^98-3^99+3^100

   3a = 3 – 3^2 + 3^3 – 3^4 + 3^5 – … + 3^99 – 3^100 + 3^101

   3a + a = ( 3 – 3^2 + 3^3 – 3^4 + 3^5 – … + 3^99 – 3^100 + 3^101) + (1-3+3^2-3^3+3^4-…+3^98-3^99+3^100)

   4a = 3^101 + 1

   => a = (3^101 + 1)/4

   Vậy a = (3^101 + 1)/4

    

   Trả lời
2. Lời giải:

   A=1-3+3^2-3^3+3^4-…+3^98-3^99+3^100

   =>3A=3-3^2+3^3-3^4+3^5-…+3^99-3^100+3^101

   =>3A+A=(3-3^2+3^3-3^4+3^5-…+3^99-3^100+3^101)+(1-3+3^2-3^3+3^4-…+3^98-3^99+3^100)

   =>4A=3^101+1

   =>A=(3^101+1)/4

   Vậy A=(3^101+1)/4

   Trả lời