Question

The sum of n terms of an ap whose first term is 6 and common difference is 40 is equal to the sum of 2n term is 40 and common difference is 6. Find n?

Answer 1

for 1st series
1st term = 6
common difference=40

now ,
sum of n terms (Sn) =n/2{2a + (n-1)d}
Sn =n/2{2 × 6 +(n-1) 40}
=n( 6 +20n -20)
=n(20n-14)

for 2nd series ,
first term = 40
common difference =6

S(2n) =2n/2{2 × 40 +(2n -1)6}
=n {80 +12n -6}
=n {74 +12n}

a/c to question,
Sn = S2n

n(20n -14) = n(74 + 12n)

20n -14 = 74 + 12n

8n =88

n=11

Answer 2

Answer:

Step-by-step explanation: so the value of n is 11 .