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## Basic geometry and measurement

### Course: Basic geometry and measurement>Unit 14

Lesson 4: Reflections

# Reflecting shapes

Let's reflect a quadrilateral across the x-axis. To do this, we find new points (A', B', C', D') by keeping the same x-coordinates and changing the y-coordinates to their opposite signs. This creates a flipped image of the original quadrilateral.

## Want to join the conversation?

• this is still complicated to me
• I suggest also looking at Algebra I concepts on transformations before coming to the High School Geometry section.
• So. Say you don't have a graph. How would you find out the coordinates with out using a graph to find the refection.
• You can use a formula. When you reflect over x-axis the coordinates are (x,-y) and when you reflect over the y-axis the coordinates are (-x,y. If you want to reflect over y=x then the coordinates are (y,x) If you want to reflect over y=-x the coordinates are (-y,-x)
• What is the X axis?
• the x axis is the principal or horizontal axis of a system of coordinates, points along which have a value of zero for all other coordinates.
• ah yes, triangle abcd
• ah yes, abcd, the easiest letters to use as variables aside from x, y and z
• it looks like a rotation not a reflection how is it a reflection
• Because as Sal showed it in the video, all the points are at the same distance from the other point. Like for point A, it is 4 above the x-axis, so he put A' (A prime) -4 from the x-axis. Does that help?
• was anyone else assigned 58 khan academy assignments by their math teacher or was it just me to suffer ?
• this still troubles me because on the khan assignment that i am working on it does not have an x or y axis or any numbers just line "l"
• The "l" line is just a reflection line.

Look at the line as if it's a mirror.
Say you want to reflect the letter A on this illustration:
-A--|----
Notice that A is 3 units away from the line.
So a reflection would look like this:
-A--|--A-

It's exactly like reflecting over Y, except we don't specify that the line is Y. It's just a reflection line.
(1 vote)
• When do you know when the shapes size changes