Use the formula for the sum of the first n integers either to evaluate the sum or to express it in closed form. a. 9 + 10 + 11 + 12 + + 400
b. 9 + 10 + 11 + 12 + + k

A) 80,164

B) S_n = (n/2)(17 + n)

Step-by-step explanation:

A) Formula for the nth term of an arithmetic progression series is;

a_n = a + (n - 1)d

a is first term of the series and in this case it is 9.

Difference is d which is 1 since they are consecutive integers.

Thus, For the last term which is 400, we have;

400 = 9 + (n - 1)1

400 - 9 = n - 1

391 = n - 1

n = 391 + 1

n = 392

Thus,there 392 terms in this sequence.

Formula for sum of an AP is;

S_n = (n/2)(2a + (n - 1)d)

S_392 = (392/2)((2 × 9) + (392 - 1)1)

S_392 = 80,164

B) The last term here is k, so we can't know exactly how many terms are in this sequence. Thus, let the number of terms be n.

Now, as above,

a = 9

d = 1

Sum of this AP sequence is;

S_n = (n/2)((2 × 9) + (n - 1)1)

S_n = (n/2)(18 + n - 1)

S_n = (n/2)(17 + n)

answer

point o.

step-by-step explanation:

i think it would be 43 movies that would have to be watched.

step-by-step explanation:

think about it. for the second plan, every month is 8 dollars. 8 x 12= 96. so thats the number for the second plan. now for the first plan, its annual meaning you have to pay 10 dollars every year. so now you have to get an equivalent amount of 86 since 96-10=86. 86 divided by 2 since every movie costs 2$ then you would have to watch 43 movies a year to get the same as the second plan. i really hoped this . have a wonderful day as well!