Question

Which is the 25th term of the arithmetic sequence where a1 = 7 and a9 = 55 ?

Answer 1

First, look at the common difference here.
We're adding 8 to get the next terms.

\( \color{Black}{\Rightarrow a_n = a_1 + (n - 1)d}\)

Let's substitute n with 25.

\( \color{Black}{\Rightarrow a_1 = 7 + 8( 25 - 1) }\)

Answer 2

there is no common difference

Answer 3

We are adding 8 every time, so 8 is the common difference.

Answer 4

how do you know to pick 8.... it only says that your first term is 7 and your 9th term is 55

Answer 5

First, count the number of terms in between, then subtract those terms, and divide those two numbers to get the common difference.

Answer 6

oooo

Answer 7

ok thanks!

Answer 8

\( \color{Black}{\Rightarrow \Large {55 - 7 \over 8} }\)

\( \color{Black}{\Rightarrow \Large {48 \over 8}}\)

Oops, its 6.

Answer 9

\( \color{Black}{\Rightarrow a_{25} = 7 + 6(25 - 1) }\)

Answer 10

\( \color{Black}{\Rightarrow 7 + 144 }\)