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Geometric relations between separable and entangled two-qubit and two-qutritquantum information states are studied. To characterize entanglement of twoqubit states, we establish a relation between reduced density matrix and theconcurrence. For the rebit states, the geometrical meaning of concurrence asdouble area of a parallelogram is found and for generic qubit states it isexpressed by determinant of the complex Hermitian inner product metric, wherereduced density matrix coincides with the inner product metric. In the case ofgeneric two-qutrit state, for reduced density matrix we find Pythagoras typerelation, where the concurrence is expressed by sum of all $2 \times 2$ minorsof $3\times3$ complex matrix. For maximally entangled two-retrit state, thisrelation is just De Gua's theorem or a three-dimensional analog of thePythagorean theorem for triorthogonal tetrahedron areas. Generalizations of ourresults for arbitrary two-qudit states are discussed

Maximally Entangled Two-Qutrit Quantum Information States and De Gua's Theorem for Tetrahedron