Answer
$=\dfrac {s\left( b\right) -s\left( a\right) }{b-a}$
Work Step by Step
$v_{av}=\dfrac {S_{b}-S_{a}}{t\left( b\right) -t\left( a\right) }=\dfrac {s\left( b\right) -s\left( a\right) }{b-a}$
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