Question

hii guys here khushi new on brainly i have a doubt in maths class 10 cbse board

how many terms if the AP 3,5,7........ must be taken so that the sum is 120

pls help me ❤️​

Answer 1

Answer:

first mark brainliest

Answer 2

10 terms

Step-by-step explanation:

A.P. = 3,5,7

a = first term = 3

d = common difference = 2

Sum of first n terms in A.P. = 5n = n/2(2a+(n-1)d)

Now we are given that the sum of n terms is 120

so, to find n:

120 = n/2 (2(3)+(n-1)2)

240 = n (6+2n-2)

240 = 4n+2n*

2n*+4n-240 = 0

n*+2n-120 = 0

n*+12n-10n-120 = 0

n(n+12)-10(n+13) = 0

(n+12) (n-10) = 0

n = -12, 10

● Since number of terms cannot be negative so, n = 10

Thus 10 terms of A.P. must be taken so that the sum is 120