Understand Line Segment Transformation Properties
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8th Grade Math › Understand Line Segment Transformation Properties
A student says: “If you rotate a segment $90^\circ$, its length might change because it points in a different direction.” Which statement best corrects the student?
The student is correct only if the rotation is about the origin.
The student is incorrect; rotations always double the length.
The student is correct; diagonal segments change length when rotated.
The student is incorrect; rotations are rigid transformations that preserve length.
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. Example: AB from (1,2) to (4,6) has length √(9+16)=5, translation by (+3,+2) gives A'(4,4) to B'(7,8) with length √(9+16)=5 unchanged. Property holds for all three transformations (move/flip/turn but don't resize). The student's claim is incorrect because rotations preserve lengths regardless of direction; for example, rotating (0,0) to (3,4) by 90° gives (0,0) to (-4,3), both lengths √[9+16]=5. The best correction is that rotations are rigid transformations that preserve length. A common error is thinking direction changes length, like claiming it doubles. To verify the concept: (1) calculate original length, (2) apply rotation rules, (3) calculate image length, (4) compare—always equal for rigid motions. All rigid transformations preserve length because they maintain distances (isometries: distance-preserving transformations)—the segment turns but not size.
A student draws segment $QR$ from $Q(-4,-2)$ to $R(-1,2)$. Then the student translates it by $(+5,-3)$ and reflects the result over the $y$-axis to get $Q''R''$. What is the length of $Q''R''$?
$\sqrt{7}$
$8$
$5$
$10$
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. For QR from (-4,-2) to (-1,2), original length √((-1 - (-4))² + (2 - (-2))²) = √(9+16) = 5; after translation (+5,-3) to (1,-5) and (4,-1), then y-axis reflection to (-1,-5) and (-4,-1), length √((-4 - (-1))² + (-1 - (-5))²) = √(9+16) = 5. Correct length is 5, as multiple transformations preserve distance. Error: incorrect order or rules, leading to wrong points and lengths like 8. Verify: (1) original length, (2) apply translation then reflection, (3) image length, (4) same. Isometries compose to preserve lengths.
Segment $NP$ has endpoints $N(2,7)$ and $P(6,2)$. It is reflected over the line $y=x$ to form $N'P'$. What is the length of $N'P'$?
$\sqrt{41}$
$\sqrt{5}$
$9$
$41$
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. For NP from (2,7) to (6,2), original length √((6-2)² + (2-7)²) = √(16+25) = √41; after y=x reflection, N'(7,2) to P'(2,6), length √((2-7)² + (6-2)²) = √(25+16) = √41. Correct length is √41, verified by swapped coordinates yielding same differences. Error: confusing with distance as sum, getting 9 instead. Verify by: (1) original distance, (2) swap x and y, (3) new distance, (4) equal. Isometries flip but keep size.
A line segment on a coordinate grid has endpoints $G(-3,1)$ and $H(1,4)$. It is translated by $(-2,+5)$ to form $G'H'$. Which statement is true?
The length changes only if the segment is diagonal.
The length becomes negative because of the $-2$ in the translation.
The length increases because both coordinates change.
The length stays the same because translations preserve distance.
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. For GH from (-3,1) to (1,4), original length √((1 - (-3))² + (4-1)²) = √(16+9) = 5; after translation (-2,+5), G'(-5,6) to H'(-1,9), length √((-1 - (-5))² + (9-6)²) = √(16+9) = 5. The true statement is that length stays the same because translations preserve distance. Errors like claiming length becomes negative from the -2 or changes only for diagonal segments ignore preservation property. To verify: (1) calculate original length, (2) add translation vector to coordinates, (3) calculate image length, (4) compare—identical. All rigid transformations are isometries, preserving distances while shifting position.
Segment $CD$ has endpoints $C(-2,5)$ and $D(4,5)$. The segment is reflected over the $y$-axis to form $C'D'$. How does the length of $C'D'$ compare to the length of $CD$?
The length doubles.
The length changes because the segment moves to negative $x$-values.
The length is cut in half.
The length stays the same.
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. For segment CD from (-2,5) to (4,5), original length is √((4 - (-2))² + (5-5)²) = √36 = 6; after reflection over y-axis, C'(2,5) to D'(-4,5), length √((-4-2)² + (5-5)²) = √36 = 6. The correct statement is that the length stays the same, as reflection flips the segment but preserves distances. A common error is thinking reflection changes length due to negative x-values, but coordinates' signs don't affect distance calculation. To verify: (1) calculate original length with distance formula, (2) apply reflection by negating x-coordinates, (3) calculate image length, (4) compare—they match. All rigid transformations preserve length because they are isometries: distance-preserving transformations that move the segment's position or orientation but not its size. Another error is assuming horizontal segments halve or double when reflected, ignoring that only position changes.
Segment $LM$ has endpoints $L(-1,-4)$ and $M(3,-4)$. It is rotated $180^\circ$ about the origin to form $L'M'$. What is the length of $L'M'$?
$8$
$4$
$-4$
$\sqrt{32}$
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. For LM from (-1,-4) to (3,-4), original length √((3 - (-1))² + (-4 - (-4))²) = √16 = 4; after 180° rotation, L'(1,4) to M'(-3,4), length √((-3-1)² + (4-4)²) = √16 = 4. The correct length is 4, as rotation inverts coordinates but preserves distance. Errors include doubling to 8 by adding differences or claiming negative length from coordinates. To verify: (1) calculate original, (2) apply (x,y) → (-x,-y), (3) calculate new length, (4) compare—same. Isometries ensure length preservation across turns.
Segment $$AB$$ has endpoints $A(1,2)$ and $B(4,6)$. The segment is translated by $(+3,+2)$ to form $A'B'$. What is the length of $A'B'$?
$7$
$5$
$\sqrt{29}$
$10$
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length $d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$ transforms to A'B' with equal length $d' = d$. Example: AB from (1,2) to (4,6) has length $\sqrt{9+16} = 5$, translation by (+3,+2) gives A'(4,4) to B'(7,8) with length $\sqrt{9+16} = 5$ unchanged. Property holds for all three transformations (move/flip/turn but don't resize). For this specific translation, original AB from (1,2) to (4,6) has length $\sqrt{(4-1)^2 + (6-2)^2} = \sqrt{9+16} = 5$, and after adding (+3,+2), A'(4,4) to B'(7,8) has length $\sqrt{(7-4)^2 + (8-4)^2} = \sqrt{9+16} = 5$, confirming preservation. The correct answer is 5, as the translation shifts the position but keeps the distance between points the same. Common errors include miscalculating the distance as $\sqrt{ (4+1)^2 + (6+2)^2 } = \sqrt{25+64} = \sqrt{89}$ or confusing with vector addition leading to wrong lengths like 7 or 10. To verify: (1) calculate original length using distance formula, (2) apply translation by adding (3,2) to each coordinate, (3) calculate image length with new points, (4) compare—they are equal. All rigid transformations preserve length because they are isometries, maintaining distances while changing position or orientation but not size.
Segment $\overline{LM}$ has endpoints $L(1,4)$ and $M(6,4)$. It is rotated $180^\circ$ about the origin to form $\overline{L'M'}$. What is the length of $\overline{L'M'}$?
1 unit
5 units
10 units
9 units
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. Example: AB from (1,2) to (4,6) has length √(9+16)=5, translation by (+3,+2) gives A'(4,4) to B'(7,8) with length √(9+16)=5 unchanged. Property holds for all three transformations (move/flip/turn but don't resize). For this 180° rotation, original LM length is √[(6-1)² + (4-4)²] = √[25+0] = 5, and after rotation, L'(-1,-4) to M'(-6,-4) has length √[(-6 - (-1))² + (-4 - (-4))²] = √[25+0] = 5, verifying preservation. The length is 5 units, as rotations keep distances. A common error is doubling differences, leading to 10 units. To verify: (1) calculate original length, (2) apply (x,y) to (-x,-y), (3) calculate image length, (4) compare—equal. All rigid transformations preserve length because they maintain distances (isometries: distance-preserving transformations)—the segment turns but not size.
Segment $\overline{EF}$ has endpoints $E(2,-1)$ and $F(2,4)$. It is rotated $90^\circ$ counterclockwise about the origin to form $\overline{E'F'}$. What is the length of $\overline{E'F'}$?
5 units
It cannot be determined without graphing.
6 units
3 units
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. Example: AB from (1,2) to (4,6) has length √(9+16)=5, translation by (+3,+2) gives A'(4,4) to B'(7,8) with length √(9+16)=5 unchanged. Property holds for all three transformations (move/flip/turn but don't resize). For this 90° counterclockwise rotation, original EF length is √[(2-2)² + (4 - (-1))²] = √[0+25] = 5, and after rotation, E'(1,2) to F'(-4,2) has length √[(-4-1)² + (2-2)²] = √[25+0] = 5, verifying preservation. The length is 5 units, unchanged by rotation. A common error is misapplying rotation rules, leading to wrong points and claiming 3 or 6 units. To verify: (1) calculate original length, (2) apply rotation using (x,y) to (-y,x), (3) calculate image length, (4) compare—equal. All rigid transformations preserve length because they maintain distances (isometries: distance-preserving transformations)—the segment turns but not size.
A line segment on grid paper has endpoints $G(-1,-3)$ and $H(3,0)$. It is reflected over the $x$-axis to form $\overline{G'H'}$. What is the length of $\overline{G'H'}$?
$5$ units
$3$ units
$7$ units
$4$ units
Explanation
This question tests understanding that rotations, reflections, and translations preserve line segment lengths—rigid transformations move segments without stretching or compressing. Rigid transformations (rotation, reflection, translation) preserve distances: segment AB with length d=√((x₂-x₁)²+(y₂-y₁)²) transforms to A'B' with equal length d'=d. Example: AB from (1,2) to (4,6) has length √(9+16)=5, translation by (+3,+2) gives A'(4,4) to B'(7,8) with length √(9+16)=5 unchanged. Property holds for all three transformations (move/flip/turn but don't resize). For this reflection over the x-axis, original GH length is √[(3 - (-1))² + (0 - (-3))²] = √[16+9] = 5, and after reflection, G'(-1,3) to H'(3,0) has length √[(3 - (-1))² + (0-3)²] = √[16+9] = 5, confirming the same. The length is 5 units, as expected for rigid transformations. A common error is using taxicab distance like |3 - (-1)| + |0 - (-3)| = 7, picking 7 units. To verify: (1) calculate original length, (2) apply reflection by negating y-coordinates, (3) calculate image length, (4) compare—identical. All rigid transformations preserve length because they maintain distances (isometries: distance-preserving transformations)—the segment flips but not size.