Question

If the first term of AP is 4 and the sum of first five term is equal to one-fourth of the sum of the next five term, then find the 15th term?​

Answer 1

Answer:

15th term = 116

Step-by-step explanation:

a= 4

Sum of an A.P = n/2 {2a + (n-1)d}

sum of first five term is equal to one-fourth of the sum of the next five term

5/2{ 2*4 + (5-1)d} = 1/4 × 10/2{2*4 + (10-1)d

5/2 {8 + 4d} = 1/4 × 5{ 8 + 9d}

40/2 + 20/2d = 1/4{ 40 + 45d)

20 + 10d= 40/4 + 45/4d

20 + 10d = 10 + 45/4d

20 - 10 = 45/4d - 10d

10 =45d - 40d /4

10 = 5/4d

Divide both sides by 5/4

10 ÷5/4 = d

10×4/5 = d

40/5 = d

8 = d

d= 8

Find the 15th term

15th term = a + (n-1)d

= 4 + (15-1)8

= 4 + (14)8

= 4 + 112

= 116

The 15th term is 116