Given:P1,P2…,P15 be 15 points on circle.
∴Total number of triangles=C315
When i+j+k=15 then possible cases are
i=1,j+k=14⇒(2,12),(3,11),(4,10),(5,9),(6,8)=5 ways
i=2,j+k=13⇒(3,10),……….,(6,7)=4 ways
i=3,j+k=12⇒(4,8),(5,7)=2 ways
i=4,j+k=11⇒(5,6)=1 way
Hence, there are total 12 ways for i+j+k=15.
∴ The number of possible triangles using the vertices Pi,Pj,Pk such that i+j+k=15 is equal to C315−12=455−12=443.