# Matrix

### Matrix

In a class on matrices, the teacher defined the concept of the orthogonal matrix to students and went further by citing its properties, saying that the module of the determinant of an orthogonal matrix is ​​always equal to 1 and also the applications in matrix decomposition and linear transformations of rotation. After the explanation, the teacher gave an example of an orthogonal matrix of order 3. One of the students managed, however, to copy only two of its lines, reproduced below:

Imagem4.gif (2.33 KiB) Viewed 313 times

At home, remembering that it was an orthogonal matrix, the student could conclude that the third line could be:

a) 2/3, -2/3 e 1/3
b) 1/3, 2/3 e 2/3
c) 2/3, 1/3 e -2/3
d) 1/3, -2/3 e 1/3
e) 2/3, 1/3 e 2/3
fismaquim

Posts: 1
Joined: Thu Jun 11, 2020 10:22 am
Reputation: 0

### Re: Matrix

Let the last row be "a b c". then, expanding the determinant on that last row,
$$a\left|\begin{array}{cc}-\frac{2}{3} & \frac{1}{3} \\ \frac{2}{3} & \frac{2}{3}\end{array}\right|-$$$$b\left|\begin{array}{cc}\frac{2}{3} & \frac{1}{3} \\ \frac{1}{3} & \frac{2}{3}\end{array}\right|+$$$$c\left|\begin{array}{cc}\frac{2}{3} & -\frac{2}{3} \\ \frac{1}{3} & \frac{2}{3}\end{array}\right|$$
$$= a\left(-\frac{4}{9}- \frac{2}{9}\right)- b\left(\frac{4}{9}- \frac{1}{9}\right)+ c\left(\frac{4}{9}+ \frac{2}{9}\right)$$
$$-\frac{2}{3}a- \frac{1}{3}b+ \frac{2}{3}c$$.

Since this question is multiple choice you can just try each of those.

HallsofIvy

Posts: 263
Joined: Sat Mar 02, 2019 9:45 am
Reputation: 98

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