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### Course: Geometry (all content)>Unit 10

Lesson 9: Old transformations videos

# Performing reflections: rectangle (old)

An older video where Sal uses the interactive widget to find the image of a rectangle under a reflection. Created by Sal Khan.

## Want to join the conversation?

• So the slope of a line never changes if it is reflected across a line of the same slope?
• But if it is reflected across a line that has a slope of 0 or opposite. Then the slope may change to being a positive or negative.
(1 vote)
• Does programming involve some math?
• Everything in the real world includes math including the room you are sitting in now
(1 vote)
• Can't we move the whole line instead of moving the points to find out if the slope is the same between the point and it's image?
• You can, it is just easier for some people to understand moving the points instead of moving the whole line. However, I do prefer moving the line and if you find it easier, I wholly recommend doing it how you feel comfortable.
• Why is the y intercept at zero? just wondering.
• Can a reflection always be used in place of a rotation and translation?
I noticed while working the problems that I could usually come up with a reflection (one step that solved the problem instead of using a rotation and translation (two steps) and I am wondering if that is generally true?
• yah i agree on your saying i think that reflection always is used in place of a rotation.
(1 vote)
• What does the equation mean? How do you use it to find the slope for the reflection?
• My question is, how am I suppose to perform reflections on pieces of paper, without the reflect tool that Khan Academy provides? Really confused, test coming up, help!
• you count from the x/y axis line to your position/dot. then, you go to the the other side of the x/y axis and count the same number of spaces towards the opposite direction.
• What does it mean to say a figure has rotational symmetry?
• what is the reflection of triangle (1,4) (3,-2) (4,2) over the x-axis?
(1 vote)
• Fractions in coordinate plane? I am Confused. 1/3x ?
• You may already know after 12 days but to be clear, a linear slope is always the (change in y) over (change in x)

(y2-y1) / (x2-x1) where y2 and x2 is a coordinate (x2, y2) at any two points and y1 and x1 are different coordinates at any two points (x1,y2)
(the 1 and 2 should be smaller numbers)
this will give you the slope.

The slope represent that any number that slope mulitplies with x will be y in a y=slope * x
(usually the linear equations looks like y = kx + b where k is slope)

for y = slope * x, the slope is -1/3 x, so if x = 3 then y = (-1*3)/3 = -1
gives coordinate (3,-1)

if x is = 9, then y = (-1*9)/3 = -3
gives coordinate (9, -3)
and
you can see this is true when change in y/change in x = slope
(-3-(-1))/(9-(3)) = (-2/6) = -1/3

hope this made sense or helped.