\( x+3 ; 2 x+6 ; 3 x+9 ; \ldots \) are the first three terms of an arithmetic sequence. (a) Determine the 10 th term in terms of \( x \). (b) Determine the \( n \)th term in terms of \( x \).
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The Deep Dive
To identify an arithmetic sequence, we need a constant difference between consecutive terms. The sequence you have is: 1st term: \( x + 3 \) 2nd term: \( 2x + 6 \) 3rd term: \( 3x + 9 \) Let's find the common difference: \( (2x + 6) - (x + 3) = 2x + 6 - x - 3 = x + 3 \) \( (3x + 9) - (2x + 6) = 3x + 9 - 2x - 6 = x + 3 \) Since the common difference is consistent, \( d = x + 3 \). (a) The \( n \)th term of an arithmetic sequence can be expressed as: \[ a_n = a_1 + (n-1) \cdot d \] Substituting in the values for \( a_1 \) and \( d \): \[ a_{10} = (x + 3) + (10-1)(x + 3) = (x + 3) + 9(x + 3) = x + 3 + 9x + 27 = 10x + 30 \] Thus, the 10th term is: \[ \boxed{10x + 30} \] (b) Now, to determine the \( n \)th term: \[ a_n = (x + 3) + (n-1)(x + 3) \] \[ = (x + 3) + (n-1)(x + 3) = (x + 3) + (nx - x + 3n - 3) = nx + 3n \] Hence, the \( n \)th term can be expressed as: \[ \boxed{nx + 3n} \]