The explicit rule of an arithmetic sequence combines the information provided by the two equations of the recursive rule into a single equation.
Recursive:& a_n=a_(n-1)+ d;
& a_1= a_1 [0.8em]
Explicit:& a_n= a_1+(n-1) d
In both of these rules, d is the common difference and a_1 is the first term. Let's start by substituting 1 for n in the given rule in order to find the value of a_1.
We know that a_1= 4, so now we need to identify the value of d. Let's take a look at the given rule.
a_n=9-5n
Notice that the coefficient of n is -5, so we know that the terms are decreasing by 5 each time. For the n^(th) term, the number of times it has decreased in relation to a_1 is given by the value of n. Therefore, the common difference d is -5. Now we have enough information to form a recursive rule for our sequence.
a_n= & a_(n-1)+( - 5);
a_1= & 4 [0.8em]
Therefore, the recursive rule for our sequence is a_1=4, a_n=a_(n-1)-5.
