Question

What is the sum of first 40 terms of 1 + 3 + 4 + 5 + 7 + 7 + 10 + 9 + ....?

A) 1010 B) 1115 C) 1030 D) 1031

Answer 1

Answer:

The answer is C

Step-by-step explanation:

The sequence has to be split into two with alternate terms:

S1=1+4+7+10+…(AP with A1=1,D1=3)

S2=3+5+7+9+...(AP with A2=3,D2=2)

Sum of 40 terms of the original series = Sum of 20 terms of S1+ Sum of 20 terms of S2

S=N[2A1+(N−1)D1]2+N[2A2+(N−1)D2]2

⟹S=[20∗((2∗1)+(19∗3))]+[20∗((2∗3)+(19∗2)]2

⟹S=10∗[59+44]=1030

HOPE IT HELPS YOU