Sn=-441 for the series 19 + 15 + 11 tn. determine the value of n.

(i’ve gotten as for as -441 = 21n - 2n^2, but i can’t seem to understand how to get to the next

The value of n is 21

Step-by-step explanation:

The series is 19 , 15 , 11 , ............

It has a constant difference between each two consecutive terms, so it is an arithmetic series.

The rule of the sum of n terms in the arithmetic series is:

Where a is the first term of the series , d is the difference between each two consecutive terms and n is the number of the terms in this series

So from the series above:

a = 19 , d = 15 - 19 = -4 ( it must be second term - first term ) ,

⇒Use factorization

(2n + 21) (n - 21) = 0

2n + 21 = 0 ⇒ 2n = -21 ⇒ n = -21/2⇒Not accepted n must be positive integer

n - 21 = 0⇒ n = 21

∴-441 is the sum of 21 terms in this series  

europa2433

europa2433

step-by-step explanation:

c.

step-by-step explanation:

because first you have to add the 4x, 9x, and the 7x together and you get 20x and then you add the last numbers up and you get 5and then you put the subtraction sign in there and that's how you get the answer.