Engineering Maths: Orthogonal Matrix

Question

Is the given last point correct?
But as i see in the given matrix sum of the product of first two columns (or) two rows is not zero. please verify.

Answer 1

the point states that the sum of the product of any two rows or columns (i.e, the product of all the elements of one row(cols)+ product of all elements of another row(cols) )

in the above example, lets take 1st and 2ns rows

product of first row elements = (cos x) (-sin x) (0) =0

product of second row elements = (-sin x) (cos x)(0) =0

their sum (1st row +2nd row) =0