2019 AMC 10B Problems/Problem 5
Problem
Triangle lies in the first quadrant. Points
,
, and
are reflected across the line
to points
,
, and
, respectively. Assume that none of the vertices of the triangle lie on the line
. Which of the following statements is not always true?
Triangle
lies in the first quadrant.
Triangles
and
have the same area.
The slope of line
is
.
The slopes of lines
and
are the same.
Lines
and
are perpendicular to each other.
Solution
Let's analyze all of the options separately.
: Clearly
is true, because a point in the first quadrant will have non-negative
- and
-coordinates, and so its reflection, with the coordinates swapped, will also have non-negative
- and
-coordinates.
: The triangles have the same area, since
and
are the same triangle (congruent). More formally, we can say that area is invariant under reflection.
: If point
has coordinates
, then
will have coordinates
. The gradient is thus
, so this is true. (We know
since the question states that none of the points
,
, or
lies on the line
, so there is no risk of division by zero).
: Repeating the argument for
, we see that both lines have slope
, so this is also true.
: This is the only one left, presumably the answer. To prove: if point
has coordinates
and point
has coordinates
, then
and
will, respectively, have coordinates
and
. The product of the gradients of
and
is
, so in fact these lines are never perpendicular to each other (using the "negative reciprocal" condition for perpendicularity).
Thus the answer is .
Counterexamples
If and
, then the slope of
,
, is
, while the slope of
,
, is
.
is the reciprocal of
, but it is not the negative reciprocal of
. To generalize, let
denote the coordinates of point
, let
denote the coordinates of point
, let
denote the slope of segment
, and let
denote the slope of segment
. Then, the coordinates of
are
, and of
are
. Then,
, and
, so slopes arent negative reciprocals of each other.
Video Solution
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Video Solution
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See Also
2019 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 4 |
Followed by Problem 6 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
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