Reading Coordinates

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ISEE Middle Level: Mathematics Achievement › Reading Coordinates

Questions 1 - 10
1

A point moves from S(-3, 2) to L(5, 2), and then from L(5, 2) to P(5, -4). What are the coordinates of the point that is exactly halfway along the total path from S to P?

(1, 2)

(4, 2)

(1, -1)

(5, -1)

Explanation

When you encounter a problem about finding a point along a path that involves multiple segments, you need to think about the total distance traveled and locate the exact halfway point along that journey.

Let's trace the path step by step. The point moves from S(-3, 2) to L(5, 2), then from L(5, 2) to P(5, -4). First, calculate the length of each segment. From S to L, you're moving horizontally from x = -3 to x = 5 while y stays at 2, so the distance is $$|5 - (-3)| = 8$$ units. From L to P, you're moving vertically from y = 2 to y = -4 while x stays at 5, so the distance is $$|2 - (-4)| = 6$$ units.

The total path length is 8 + 6 = 14 units, so the halfway point is at 7 units from the start. Since the first segment (S to L) is 8 units long, the halfway point falls somewhere on this first segment. You need to move 7 units along the horizontal line from S(-3, 2). Moving 7 units right from x = -3 gives you x = -3 + 7 = 4, while y remains 2. Therefore, the halfway point is (4, 2).

Choice A) (1, 2) represents moving only 4 units from S, not halfway. Choice B) (1, -1) incorrectly assumes you need to find the midpoint between the endpoints S and P, ignoring the actual path. Choice C) (5, -1) makes the same midpoint error. Choice D) (4, 2) is correct.

Remember: "Halfway along the path" means half the total distance traveled, not the midpoint between start and end coordinates.

2

A point K is at (-5, 3). It is reflected across the y-axis to create point L. Then, point L is reflected across the x-axis to create point M. What are the coordinates of point M?

(5, -3)

(-5, 3)

(5, 3)

(-5, -3)

Explanation

First, reflect K(-5, 3) across the y-axis. A reflection across the y-axis changes the sign of the x-coordinate. So, L has coordinates (5, 3). Next, reflect L(5, 3) across the x-axis. A reflection across the x-axis changes the sign of the y-coordinate. So, M has coordinates (5, -3).

3

Point P has coordinates (a, b) and lies in Quadrant II. Point Q is the reflection of point P across the x-axis. In which quadrant does point Q lie?

Quadrant I

Quadrant III

Quadrant IV

On an axis

Explanation

A point in Quadrant II has a negative x-coordinate (a < 0) and a positive y-coordinate (b > 0). When a point is reflected across the x-axis, its x-coordinate stays the same and the sign of its y-coordinate changes. So, point Q will have coordinates (a, -b). Since a is negative and -b is negative (because b was positive), point Q has a negative x-coordinate and a negative y-coordinate. Points with two negative coordinates lie in Quadrant III.

4

Three consecutive vertices of a parallelogram MNOP are M(-2, -2), N(3, -2), and O(5, 1). What are the coordinates of the fourth vertex, P?

(0, 1)

(0, -3)

(10, 1)

(-4, -5)

Explanation

In a parallelogram MNOP, the vector from M to N must be equal to the vector from P to O. The vector MN is found by subtracting M's coordinates from N's: (3 - (-2), -2 - (-2)) = (5, 0). Let P be (x, y). The vector PO is (5-x, 1-y). Setting these equal: 5-x = 5 implies x=0, and 1-y = 0 implies y=1. So P is (0, 1). Alternatively, vector NO must equal vector MP. Vector NO is (5-3, 1-(-2)) = (2, 3). Vector MP is (x - (-2), y - (-2)) = (x+2, y+2). So x+2=2 implies x=0, and y+2=3 implies y=1. The coordinates of P are (0, 1).

5

If a point (x, y) is in Quadrant IV, which of the following points must be in Quadrant II?

(-y, -x)

(x, -y)

(y, x)

(-x, y)

Explanation

When you see coordinate plane questions, remember that each quadrant has a specific sign pattern. Quadrant I has positive x and y values (+,+), Quadrant II has negative x and positive y (-,+), Quadrant III has both negative (-,-), and Quadrant IV has positive x and negative y (+,-).

Since point (x, y) is in Quadrant IV, we know that $$x > 0$$ and $$y < 0$$. To find which transformed point lands in Quadrant II, we need the result to have a negative x-coordinate and positive y-coordinate.

Let's check each option systematically. For choice A, (x, -y): since x is positive and y is negative, -y becomes positive, giving us (+,+), which places the point in Quadrant I, not II. For choice B, (-x, y): since x is positive, -x is negative, and y is already negative, this gives us (-,-), placing it in Quadrant III. Choice D, (-y, -x): since y is negative, -y is positive, and since x is positive, -x is negative, giving us (+,-), which is Quadrant IV again.

Choice C, (y, x), works perfectly. Since y is negative (from our original Quadrant IV point), it becomes the new x-coordinate, making it negative. Since x is positive, it becomes the new y-coordinate, staying positive. This gives us (-,+), which defines Quadrant II.

Remember this strategy: when dealing with coordinate transformations, substitute the known signs from the original quadrant into each answer choice to determine where the new point lands.

6

A robot begins at the origin (0, 0). It follows a sequence of moves: 7 units east, 4 units north, 3 units west, and finally 8 units south. What are the final coordinates of the robot's location?

(10, -4)

(4, -4)

(10, 12)

(4, 12)

Explanation

We track the changes to the x and y coordinates. East/west movements affect the x-coordinate, and north/south movements affect the y-coordinate. East and north are positive directions; west and south are negative. The final x-coordinate is 0 + 7 - 3 = 4. The final y-coordinate is 0 + 4 - 8 = -4. The robot's final location is (4, -4).

7

A point starts at (8, -6). It is translated 4 units up. Then, its new coordinates are both halved. What are the final coordinates of the point?

(4, -3)

(8, -4)

(4, 1)

(4, -1)

Explanation

When you encounter coordinate transformation problems, work through each step systematically in the order given, applying one transformation at a time to avoid errors.

Let's start with the point at $$(8, -6)$$ and apply the first transformation: translating 4 units up. Moving up means adding to the y-coordinate, so we get $$(8, -6 + 4) = (8, -2)$$.

Next, we halve both coordinates of our new point $$(8, -2)$$. Halving means dividing each coordinate by 2: $$x = \frac{8}{2} = 4$$ and $$y = \frac{-2}{2} = -1$$. This gives us the final coordinates $$(4, -1)$$, which is choice B.

Let's examine why the other answers are incorrect. Choice A $$(4, -1)$$ gets the x-coordinate right but shows $$y = 1$$, which would happen if you incorrectly added 4 instead of subtracting when translating up from $$-6$$, or if you forgot the negative sign when halving $$-2$$. Choice C $$(4, -3)$$ correctly halves the x-coordinate but shows $$y = -3$$, which occurs if you halve the original y-coordinate $$(-6 ÷ 2 = -3)$$ without first applying the upward translation. Choice D $$(8, -4)$$ appears to show the coordinates after translation and halving in the wrong order—perhaps halving first to get $$(4, -3)$$, then translating up to get $$(4, -1)$$, but somehow ending up with the wrong values entirely.

Always perform transformations in the exact sequence given in the problem. Write down your coordinates after each step to track your progress and catch mistakes early.

8

A shape drawn on a coordinate plane is symmetric with respect to the origin. If the point (-4, 9) is on the shape, which of the following points must also be on the shape?

(4, -9)

(9, -4)

(4, 9)

(-4, -9)

Explanation

When you see a question about symmetry with respect to the origin, you're dealing with point symmetry (also called rotational symmetry of 180°). This means that if you rotate the entire shape 180° around the origin, it looks exactly the same.

For any point $$(x, y)$$ on a shape that's symmetric with respect to the origin, the point $$(-x, -y)$$ must also be on the shape. This is because rotating a point 180° around the origin changes both coordinates to their opposites.

Starting with the given point $$(-4, 9)$$, you need to find its symmetric counterpart by negating both coordinates: $$(-(-4), -(9)) = (4, -9)$$. This confirms that choice D is correct.

Let's examine why the other options are wrong. Choice A $$(9, -4)$$ swaps the coordinates and negates one of them - this represents a reflection across the line $$y = -x$$, not point symmetry. Choice B $$(4, 9)$$ only negates the x-coordinate, which represents reflection across the y-axis. Choice C $$(-4, -9)$$ only negates the y-coordinate, representing reflection across the x-axis.

Remember this key pattern: origin symmetry always means "flip both signs." When you see point symmetry questions on the ISEE, immediately apply the rule $$(x, y) \rightarrow(-x, -y)$$. Don't confuse it with line reflections, which only change one coordinate or swap them entirely.

9

A point P has coordinates (k, 3k - 1). If k = 2, the point is translated 5 units to the left and 2 units up. What are the new coordinates?

(7, 3)

(-3, -10)

(2, 5)

(-3, 7)

Explanation

This question tests coordinate geometry and transformations. When you see a problem involving translations, remember that you're shifting points on a coordinate plane using specific rules.

First, substitute $$k = 2$$ into the coordinates $$(k, 3k - 1)$$. For the x-coordinate: $$k = 2$$. For the y-coordinate: $$3k - 1 = 3(2) - 1 = 6 - 1 = 5$$. So point P starts at $$(2, 5)$$.

Next, apply the translation. "5 units to the left" means subtract 5 from the x-coordinate, and "2 units up" means add 2 to the y-coordinate. The new x-coordinate is $$2 - 5 = -3$$. The new y-coordinate is $$5 + 2 = 7$$. Therefore, the new coordinates are $$(-3, 7)$$.

Looking at the wrong answers: Choice A gives $$(2, 5)$$, which is the original point before translation—you'd get this if you forgot to apply the transformation. Choice B gives $$(-3, -10)$$, which has the correct x-coordinate but gets the y-coordinate by subtracting instead of adding ($$5 - 2 = 3$$, then somehow getting $$-10$$)—this suggests confusion about direction. Choice C gives $$(7, 3)$$, which results from moving right instead of left ($$2 + 5 = 7$$) and down instead of up ($$5 - 2 = 3$$)—both directions are reversed.

Remember the translation rules: left means subtract from x, right means add to x, up means add to y, and down means subtract from y. Always double-check which direction each movement should go.

10

A point is located in the coordinate plane. Its x-coordinate is the greatest common factor of 12 and 18. Its y-coordinate is the least common multiple of 4 and 6. What are the coordinates of the point?

(3, 12)

(2, 36)

(12, 6)

(6, 12)

Explanation

This question tests two fundamental concepts in number theory: greatest common factor (GCF) and least common multiple (LCM). When you see these terms together, you're working with finding shared factors and multiples of given numbers.

To find the x-coordinate, you need the GCF of 12 and 18. Start by listing the factors of each number. The factors of 12 are: 1, 2, 3, 4, 6, 12. The factors of 18 are: 1, 2, 3, 6, 9, 18. The greatest factor they share is 6, so the x-coordinate is 6.

For the y-coordinate, you need the LCM of 4 and 6. List the first several multiples of each number. Multiples of 4: 4, 8, 12, 16, 20... Multiples of 6: 6, 12, 18, 24... The smallest multiple they share is 12, so the y-coordinate is 12. The coordinates are (6, 12).

Looking at the wrong answers: Choice A gives (2, 36). Here, 2 is a common factor of 12 and 18, but not the greatest one, and 36 is a common multiple of 4 and 6, but not the least. Choice B shows (12, 6), which reverses the coordinates—a common error when working quickly. Choice D gives (3, 12). While 3 is a common factor of 12 and 18, it's not the greatest common factor.

Remember: GCF is always smaller than or equal to the smallest given number, while LCM is always greater than or equal to the largest given number. This can help you quickly eliminate unreasonable answers.

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