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the sequences are:

1.......................[ $1^2$ ]

4.......................[ $2^2$ ]

9.......................[ $3^2$ ]

16....................[ $4^2$ ]

so, the 18th term will be= $18^2=324$

clearly it is a sequence of squares so 18th term will be 18square=324

The eighteenth term is the eighteen square which is $\color{#69047E}{\boxed{324}}$ .

Un = n^2

U18 = 18^2 = 324.

it is a simple series the 18th term's sq will be the ans that is equal to 324

The series is like this 1square, 2square, 3square and so on 18th term must be 18square ie. 324

Given series shows that second number is square of 2,3rd is square of 3 and so on,so 18th number will be square of 18 ,that is 324,Ans. K.K.GARG,India