Answer:
the answer is 384
Step-by-step explanation:
a1 = 21, a2 = 23, a3 = 25
an = 43
d = a2 - a1 = 23 - 21 = 2
d = a3 - a2 = 25 - 23 = 2
Here, common difference is same everywhere
So,
[tex] a_{n} = a + (n - 1) \times d[/tex]
[tex]43 = 21 + (n - 1) \times 2[/tex]
[tex]43 - 21= 2n - 2[/tex]
[tex]2n - 2 = 22[/tex]
[tex]2n = 24 [/tex]
[tex]n = 12[/tex]
Now,
[tex]s _{n} = \frac{n}{2} (a + a_{n}) \\ [/tex]
[tex]s _{12} = \frac{12}{2} (21+ 43) \\ [/tex]
[tex]s _{12} = 6 (64) \\ [/tex]
[tex]s _{12} = 384 \\ [/tex]
Answer: three hundred eighty four
Step-by-step explanation:
