Question

Calculate s5 for the sequence defined by

Answer 1

ANSWER

$S_5= 42.5$

EXPLANATION

The general term of the arithmetic sequence is

$a_n=1+\frac{5}{2}n$

For the first term,we substitute $n=1$ to get,

$a_1=1+\frac{5}{2}\times 1$

$a_1=1+2.5=3.5$

Similarly

$a_5=1+\frac{5}{2}\times 5$

$a_5=13.5$

The sum of the first n-terms is given by the formula,

$S_n=\frac{n}{2}(a_1+l)$

where l is the last term.
In this case

$l=a_5=13.5$

We substitute these values to get,

$S_5= \frac{5}{2} (3.5 + 13.5)$

$S_5= \frac{5}{2} (17)$
$S_5=42.5$



We could have also used the formula,

$S_n=\frac{n}{2}(2a_1+(n-1)d)$

$S_5=\frac{5}{2}(2(3.5)+(5-1)2.5)$

$S_5=42.5$

Answer 2

Answer:

42.5 is the one