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Properties of arithmetic operations are important in mathematics and play a significant role in elementary school students' learning of mathematics. At the same time, it is quite challenging to help students learn arithmetic operational properties beyond memorization, so that they know when and how to use a specific arithmetic property. This paper reports a study containing two parts to explore students' learning of the Commutative Property of Addition (CPA) as related to the content presentation in textbooks. Part one is to assess selected students' performance on CPA at the end of Grade two, and part two is to explore possible contributions of textbooks to students' performance in Shanghai. Firstly, we evaluated the performance of 41 second-graders on CPA by collecting data through a paper-and-pencil test, which was followed up with individual interviews. Through analyzing how textbooks present and organize this topic, we sought to better understand from a curriculum standpoint how students' performance on CPA is impacted by textbooks. Results revealed that Shanghai textbooks make a positive contribution to students' learning performance. The findings from this study help to shed light on how those textbooks aid student learning, and provide an example for further reflection on student learning of arithmetic operational properties in different curricula.

The description of the representations of the indefinite orthogonal groups O(p,q) is presented. From the formulas for the O(p,q) action, the structure of the composition series is found to depend on the parities of p and q. The formulas are applied to determine which representations carry invariant Hermitian forms and which constituents of the representations are unitary. The role of these representations in the general theory of representations of semisimple Lie groups. semisimple Lie groups is discussed.

The article offers information on a proof of the Law of Cosines using squares constructed on sides of the triangle. It discusses the configuration consisting of the original triangle and the triangle defined by the centers of these squares. It mentions that the Law of Cosines is a direct generalization of Euclid's proof of the Pythagorean Theorem.