Question

The first term of an AP is 2.If S5 is1÷4 of next 5 terms then find s20​

Answer 1

Answer:

a = 2

S5 = 1/4 (S10 - S5)

S5 = 5/2 [2(2) + 4d]

     = 5/2 [4 +4d]

     = 5x2 [1+1d]

    =10+10d.

S10 = 10/2{2a+9d}

      = 5[4+9d]

     = 20+45d

(S10 - S5) = 20+45d -(10+10d)

               = 10+35d

S5 = 1/4 ( S10- S5 )

10+10d = 1/4 ( 10+ 35d)

40+40d = 10+ 35d

5d = -30

d = -6

S20 = 20/2[2a+19d]

        = 10{4+ (-114)}                                            |  19 d = 19 x -6 = -114

        = 10[-110]

        = -1100