Question

If the sum of n terms of an AP is 3n2 +5n then which term is 164?
(a) 26th b)27th
(c) 28th
(d) none of these
If radii of two concentric circles are 6 cm and 10 cm, then length of the chord of one circle wh
tangent to the other circle is
(a) 5 cm (b) 8 cm (c) 12cm
(d) 16 cm
​

Answer 1

Answer: b) 27th

Step-by-step explanation:

sum = 3n2 + 5n

      a = 3(1)2 +5(1)

       a   = 8  

a1 + a2 = 3(2)2 + 5(2)

  8 + a2 = 22

        a2  = 22 - 8

        a2   = 14

d= a2 - a1

  = 14-8

   = 6

an= 164

an= a + (n-1)d

    = 8 + (n-1)*6

164-8 = (n-1)*6

      156/6 = n-1

       n-1 = 26

       n= 27

hope this helped