Question

the first term of ap is -5 and last term is 4 .if sum is 120, find n and d​

Answer 1

Answer:

the value of n= - 240 hope it is helpful 2 u

Answer 2

Correct question :

the first term of ap is -5 and last term is 45 .if sum is 120, find n and d

Solution :

Let us consider an A.P. whose first term and common difference are a and d respectively.

Here, a = -5, a_n = 45, S_n = 120

Now, S_n = n/2[2a+(n - 1)d] = n/2[a+a+(n-1)d]

s_n = n/2 [a + a_n][a_n = last term]

120= n/2 (-5 + 45)

120 = n/2 x 40

n = 120x2/40 = 6

n = 6

Hence, the number of terms = 6

Now, a_n = a + (n-1)d

45 = -5 + (6-1)d

45 = 5 - 5d

5d = 50

d = 50/5

d = 10

=> Hence, the common difference and the number of terms in A.P. are 10 and 6 respectively.