Question

Q1.If the sum of 'n' terms of A.P 2, 5, 8,
is 155, then the number of terms is:

Answer 1

Explanation:

Given

sum of n terms of AP 2,5,8 is 155

To Find

we have to find the number of terms

$\sf\huge {\underline{\underline{{Solution}}}} </p><p>Solution</p><p> </p><p>$

AP : 2,5,8 ..

here ,first term (a) is 2

second term (a2) = 5

common difference (d) = a2-a1

d= 5-2= 3

Now, using formula

an= a+(n-1)d

an= 155

a= 2

d= 3

155= 2+(n-1)3

155=2+3n-3

155= 3n-1

3n= 156

n= 156/3= 52

Hence, number of term is 52

extra information=>

If n term is given and to find sum we use :

Sn= n/2[2a+(n-1)d

if last term is given and to find sum we use :

Sn= n/2( a+l)