# Finding reflection of a matrix

To 2 decimal places, what is the value of the lower-right entry in the reflection matrix $Q_a$if a = 1.05?

Not even sure where to begin, is there a formula?

This is what I could find in my textbook

-
"in the reflection matrix $Q_a$" - what is the definition of $Q_a$ ? – Belgi Oct 21 '12 at 15:37
Thats it the question I was given. – JackyBoi Oct 21 '12 at 15:45
@Belgi this has to do with transformations – JackyBoi Oct 21 '12 at 15:46
There is no universal definition of $Q_a$. You're going to have consult your textbook, class notes, or your instructor to determine the meaning of this symbol before proceeding. – Michael Joyce Oct 21 '12 at 16:02
@MichaelJoyce I have edited my question please take a look. – JackyBoi Oct 22 '12 at 1:25

Okay, well the lower right entry of the matrix $q_a$ is $-\cos(2a)$. So plug in $a = 1.05$ and use a calculator to get an approximate value for $-\cos(2.10) \approx 0.50485$. Be sure to use 'radian' mode on your calculator. (Always assume angle measures for trigonometric functions are in radians unless explicitly told otherwise.)