Question

How many terms of the ap -6,-11/2,-5 make the sum-25

Answer 1

Answer:

5th or 20th

Step-by-step explanation:

Sum of n terms of AP = Sₙ = n/2[2a + (n-1)d]

a = -6, Sₙ = - 25, d = -11 /2+6 = 1/2

-25 = n/2[ - 12 + (n-1) 1/2]

- 25 = n/2[[-24 + n - 1 /2]

- 25= n/4[n - 25]

n² -25n + 100 = 0

n² -20n - 5n + 100 = 0

n(n - 20) - 5 (n-20) = 0

(n-5) (n-20) = 0

n = 5 or 20.