yields the coordinates and yeilds the coordinaes . So every value in the is the mirror image of the outputs. That's why we say the is the reflection through the of . Make sense?
"f(x)" is just another name for "y". If you take a graph's y-values and change their signs, then you're moving the points to the opposite side of the x-axis. In other words, you're flipping the graph upside-down.