The number of triangles formed by the vertices of decagon such that each triangle and decagon have.

n= No. of vertices of decagon = 10. No. of  $\Delta$  less
(i) with exactly one side common =  $n(n-4)=60$
(ii) with exactly two sides common = n=10
(iii) with exactly there sides common = 0
(iv) at least one side common = 60+10=70.