Question

given a 3 = 15, s10= 125. Find d&a
10​

Answer 1

Answer:

a = 651/25

d = - 138/25

Explanation:

Given : a₃ = 15

S₁₀ = 12

To Find :

d and a

Formula Used :

nth term an = a + (n - 1) d

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

a₃ = 15

a + 2d = 15.

a = 15 - 2d. . . . . . . (1)

S₁₀ = 12

10/2 [ 2a + 9d] = 12

5 [ 2a + 9d ] = 12

5 [ 2(15 - 2d) + 9d] = 12

10(15 - 2d) + 45d = 12

150 - 20d + 45d = 12

25 d = 12- 150

25d = -138

d = - 138/25

From (1)

a = 15 - 2 (-138/25)

= 375 + 276 /25

= 651/25