Question

find the sum of 15 terms of an ap whose first and last term 5 and 75

Answer 1

a = 5

n = 15

L = 75


Sn = n/2 ( a + L)

Sn = 15 /2 ( 5 + 75)

= 15 × 80 /2

= 15 × 40

= 600

Required sum = 600

Answer 2

Heya !!




First term ( a ) = 5


Last term ( d ) = 75


sn = n/2 × ( a + l )


s 15 = 15/2 × ( 5 + 75 )



=> 15/2 × 80



=> 15 × 40



= 600