I believe the radius is 5 <span />
Answer:
He needs 7 more consecutive successful first serves to raise his first serve percentage to 60%.
Step-by-step explanation:
After n consecutive serves, his total number of serves is going to be n+8, since he has already served 8 times. In the best case, his number of successful first serves is n+2.
His percentage of succesful first serves is the division of the number of succesful first serves divided by the total number of serves. So

We want
. So







He needs 7 more consecutive successful first serves to raise his first serve percentage to 60%.
Let's assume that a,b&c are in one straight line, so cannot form a triangle with each other. Now, total possible Triangle that can be formed choosing any 3 points without any colinear constraint is 8C3 = 56
<em><u>The common difference d is </u></em>
<em><u>The first term is</u></em>
<em>so , the first term is -13 </em>