Question

if an =3-4n show that A1. A2. A3. ......... form an ap also find s20​

Answer 1

Step-by-step explanation:

An = 3 - 4n

A1 = 3 - 4(1) = 3 - 4 = -1

A2 = 3 - 4(2) = 3 - 8 = -5

A3 = 3 - 4(3) = 3 - 12 = -9

a = -1 , d = -5 + 1 = -4

S(20) = 20/2 { 2(-1) + (20-1)(-4) }

= 10 { -2 + 19(-4) }

= 10 { -2 - 76 }

= -78 × 10 = -780