$S_{n} = \frac{n}{2} (4n + 20) \\ \\S _{n - 1} = \frac{(n - 1)}{2} (4n - 4 + 20) \\ \\ S _{n - 1} = \frac{(n - 1)}{2} (4n + 16) \\ \\S _{n - 1} = \frac{(n - 1)(4n + 16)}{2} \\ \\ { \boxed{S _{n - 1} = {2 {n}^{2} + 6n - 8}}} \\$

(b).

from general equation:

$S _{n - 1} = \frac{(n - 1)}{2} (4n + 16)$

first term is 4n

common difference:

$16 = \{(n - 1) - 1 \}d \\ 16 = (n - 2)d \\ d = \frac{16}{n - 2}$

8 0

3 years ago

What is the best way to learn mathematics easily??

emmasim [6.3K]

I would recommend going to school

Or get a tutor

Or ask your parents

4 0

3 years ago

Factor completely: 4x2 + 25x + 6

IceJOKER [234]

(x + 6)(4x + 1)
x times 4x = 4x^2
x times 1 = x
6 times 4x = 24x
6 times 1 = 6
<span>And finally, by adding like terms, 4x^2 + 25x + 6

I hope this helps you! Good luck :)</span>

7 0

3 years ago

What is the value of x in the triangle?​

What is the value of x in the triangle?