Question 1178123
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            Tutor @amarjeeth is not right,  and his answer is not correct.




If to accept the conception,  that a ray is determined by a pair of points,

then these pairs must be considered as  ORDERED  (an order does matter),


and the number of different pairs and different rays will be   10*9 = 90   (not  45  (!) ).



In this problem,  we work with  PERMUTATIONS  of  10  points, taken  2  at a time,  and  NOT  with the  COMBINATIONS  (!).



Again,  the order of points in pairs  DOES  MATTER  in this problem (!).



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Solved  (correctly).