令G为具有10个顶点的连通平面图。如果每个面上的边数为3，则G中的边数为________________。
(A) 24
(B) 20
(C) 32
(D) 64答案： (A)
说明：欧拉公式指出，如果在平面中绘制了有限的，连通的平面图，而没有任何边缘相交，则
v − e + f = 2.
v -> Number of vertices
e -> Number of edges
f -> Number of faces
As per the question
v = 10
And number of edges on each face is three
Therefore, 2e = 3f [Note that every edge is
shared by 2 faces]
Putting above values in v − e + f = 2
10 - e + 2e/3 = 2
e = 3*10 - 6 = 24这个问题的测验