Question

If the end term of an ap is - 1 4 9 is 129 find the value of n

Answer 1

Question :-

A.P = -1, 4, 9

End term (nth term) = 129

Find the value of 'n'

Answer :-

Now, from the given A.P

a (first term) = -1

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

nth term = 129

Now, using nth term formula

nth term = a + (n-1)d

-» 129 = -1 + (n-1)(5)

-» 129 + 1 = (n-1)(5)

-» 130 = (n-1)(5)

-» ${\dfrac{130}{5}}$ = n - 1

-» 26 = n - 1

-» n = 27

$\huge\bf\red{\mid{\overline{\underline{n = 27}}}\mid}$