Question

The first term of an A.P. is 2. Sum of the first 5 terms is one fourth of the sum of the next five terms. Prove that
a20=-112. Find S20

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Answer 1

Answer:

Step-by-step explanation:

a=2

S5=1/4(S10)

Hence, after solving,  

5(S5)=S10

5( 5/2(2*2+4d)) = 5(4+9d)

10d + 10 = 9d + 4

Therefore, d= -6

a20 = a+19d

= 2+ 19(-6)

= 2-114

=-112

Hence proved.