  An interesting monogamy equation with the form of Pythagorean theorem is
found for $2\otimes 2\otimes n$-dimensional pure states, which reveals the
relation among bipartite concurrence, concurrence of assistance, and genuine
tripartite entanglement. At the same time, a genuine tripartite entanglement
monotone as a generalization of 3-tangle is naturally obtained for $(2\otimes
2\otimes n)$- dimensional pure states in terms of a distinct idea. For mixed
states, the monogamy equation is reduced to a monogamy inequality. Both results
for tripartite quantum states can be employed to multipartite quantum states.
