Question

Q. If the nth of an AP -1,4,9,14...........is 129 . Find the value of n.
PLZZ HELP ME ANYONE WHO KNOW THE CORRECT ANSWER PLZZZZ ☺️☺️​

Answer 1

An=a+(n-1)d

where

An= last term of an AP

a = first term of an AP

n= finding term of an AP

d= common difference

Step-by-step explanation:

So,

An=a+(n-1)d

129= (-1) + (n-1)(5) [ d= a2-a1]

129+1= (n-1)5

130/5= n-1

26+1=n

hence, n= 27

I hope this wll help u!!

Answer 2

${\huge{\underbrace{\rm{Answer\checkmark}}}}$

A.P. → -1 , 4 , 9 , 14 , ... , 129

As , u can see ..

a (first term) = -1

d (common Difference) = 4-(-1) = 5

An or L (last term) = 129

Using the Formula -

→ An/l = a + (n-1) d

→ 129 = -1 + (n-1) × 5

→ 129 + 1 = (n-1)×5

$\rm \mapsto \: \cancel{130} = (n - 1) \cancel5$

$\mapsto \rm \: 26 = n - 1$

$\mapsto \rm \: n \: = 26 + 1$

$\mapsto \boxed{\rm \: n = 27}$