Questions: The first four terms of an arithmetic sequence are -11, -5, 1, 7. What is the equation for an?
A an = -11(n-1)-6
B an = 6(n-1)-11
C an = -11(n-1)+6
D an = -6(n-1)-11
Transcript text: The first four terms of an arithmetic sequence are $-11,-5,1,7$. What is the equation for $a_{n}$ ?
A $a_{n}=-11(n-1)-6$
B $a_{n}=6(n-1)-11$
C $a_{n}=-11(n-1)+6$
D $a_{n}=-6(n-1)-11$
Solution
Solution Steps
Step 1: Identify the first term and common difference
The first term of the arithmetic sequence is \( a_1 = -11 \).
The common difference \( d \) is calculated as:
\[ d = a_2 - a_1 = -5 - (-11) = 6. \]
Step 2: Write the general formula for an arithmetic sequence
The general formula for the \( n \)-th term of an arithmetic sequence is:
\[ a_n = a_1 + (n-1)d. \]
Step 3: Substitute the known values into the formula
Substitute \( a_1 = -11 \) and \( d = 6 \) into the formula:
\[ a_n = -11 + (n-1) \cdot 6. \]
Simplify the equation:
\[ a_n = 6(n-1) - 11. \]
Step 4: Compare with the given options
The equation \( a_n = 6(n-1) - 11 \) matches option B.