Question

How many terms of the series 1+ 5+9+...........must be taken so that theirs sum is 120​

Answer 1

Answer:

Bhai ap ka formula a+(n-1)d

a=1

difference=4

Step-by-step explanation:

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Answer 2

★$\large\color{red} \mathrm{Hey \: There!}$★

Let number of terms to be taken = n

Thus,

.

Sn = n/2{2a + (n-1)d}

120 = n/2{2 + (n-1)4}

240 = n(2 +4n - 4}

240 = n(4n-2)

120 = n(2n - 1)

120 = 2n^2 - n

2n^2 - n -120 = 0

2n^2 - 16n + 15n - 120 = 0

2n(n - 8) + 15(n - 8) = 0

(2n + 15)(n - 8) = 0

=> n = -15/2 ........... or ............ n = 8

Since, n is number of terms. Hence, it can't neither be a -ve integer nor a fractional value.

.

Therefore, the correct answer is n = 8.