Question

All students of a class decided to present a program in the annual fuction .They

were standing in groups with different numbers (which make an AP).In first

group there are 5 members and in the last group(fifth group) there are 17

members. Find total number of students of that class.​

Answer 1

According to the question

a1 = 5

a5 = 17

d = am-an

m-n

d = 17-5

5-1

d = 12

4

d = 3

now, using S = n × (a + l)

2

S = 5 × (5 + 17)

2

S = 5 × 22

2

S = 5 × 11

S = 55

Thus, there are 55 no. of students in that class.