Question

if n term of A.P is An=4-7n find the sum of first 20th term​

Answer 1

Answer:

Sum of first 20 terms is -1270

Step-by-step explanation:

Given Aₙ=4-7n

A₁=4-7(1)=-3

A₂=4-7(2)=-10

A₃=4-7(3)=-17

and so on

First term a=-3

Common difference =a2-a1

                                =-10+3

                                 =-7

Sum of first 20 terms is given by

S₂₀=$\frac{n}{2}$(2a+(n-1)d)

    =10(2(3)+19(-7))

   =10(6-133)

    =10(-127)

    =-1270

Sum of first 20 terms is -1270