Answer:

SOLUTION

:

From the problem

A1 = 15

A2 = 13

We get the common difference:

D = A2 - A1

D = 13 - 15

D = -2

\begin{gathered} \\ \end{gathered}

Now , we get the sum by substituting the values to the formula:

Sn = n/2 [ 2a1 + (n-1)d ]

S(6) = 6/2 [ 2(15) + (6-1)-2 ]

S(6) = 3 [ 30 + (5)-2 ]

S(6) = 3 [ 30 + (-10) ]

S(6) = 3 ( 20 )

S(6) = 60

\begin{gathered} \\ \end{gathered}

\large\underline{ \mathcal {ANSWER}}:

ANSWER

:

60 kilometers