Question

If the sum of n terms of an

a.P is 2 + 5, then find the 4th term.

Answer 1

Answer:

Step-by-step explanation:

a3=a+2d

15=a+2d               (1)

S10=10/2[2a+9d]

125=5[2a+9d]

25=2a+9d.           (2)

Multiply (1) by 2

2a+4d=30

2a+9d=25

-    -      -

_________

-5d=5.          =>d = -1

                    =>a = 17

Now,

a10 = a+9d

     = 17+(-9)

     = 17-9.  =>8

Sn= n/2[2a+(n-1)d]

    = n/2[34-n+1]

     =n/2[35+n]

     = 35n+n^2/2