Question

find the sum of first n terms of an ap,whose nth term is 5n-1.hence find the sum of first 20 terms​

Answer 1

Answer:

Step-by-step explanation:

an = 5n-1

a1 = 5(1) - 1

=4

a2 = 5(2) - 2= 9

now ; we have

    a= 4

    d= 9-4=5

sn =n/2 {2a + (n- 1)d}

=20/2{8 +19 (5)}

= 10{8+ 95}

=10{103}

=1030