Question

2.Show that the terms of are in A.P .Find the sum of the 1st twenty terms of the series and the least value of n for which the sum to n terms exceeds 400.

Answer 1

Step-by-step explanation:

Then,

a1 = a

a2 = a + d

a3 = a + 2d

a4 = a + 3d

………..

………..

an = a + (n - 1)d

Now,

S = a1 + a2 + a3 + ………….. + an−1 + an

S = a + (a + d) + (a + 2d) + (a + 3d) + ……….. + {a + (n - 2)d} + {a + (n - 1)d} ……………….. (i)