Chapter 14: Problem 1260
The circumcentre of the triangle formed by the lines \(\mathrm{x}+\mathrm{y}=0, \mathrm{x}-\mathrm{y}=0\) and \(\mathrm{x}-7=0\) is \(\ldots \ldots\) (a) \((7,0)\) (b) \((3.5,0)\) (c) \((0,7)\) (d) \((3.5,3.5)\)
Short Answer
Expert verified
The circumcentre of the triangle formed by the lines $\mathrm{x}+\mathrm{y}=0,
\mathrm{x}-\mathrm{y}=0$ and $\mathrm{x}-7=0$ is $(3.5, -3.5)$.
Step by step solution
01
Find the vertices of the triangle
To find the vertices, we need to solve the given linear equations in pairs:
1. x + y = 0 and x - y = 0
2. x + y = 0 and x - 7 = 0
3. x - y = 0 and x - 7 = 0
Solving the first pair of equations, we get:
x = 0 and y = 0, which give us the first vertex A(0, 0).
Solving the second pair of equations, we get:
x = 7 and y = -7, which give us the second vertex B(7, -7).
Solving the third pair of equations, we get:
x = 7 and y = 7, which give us the third vertex C(7, 7).
02
Calculate the midpoints and slopes of two sides
Consider two sides AB and AC.
Find the midpoints of these two sides:
Midpoint of AB, M1 = \(\left(\frac{A_x + B_x}{2}, \frac{A_y + B_y}{2}\right)\) = \(\left(\frac{0+7}{2}, \frac{0-7}{2}\right)\) = \((3.5, -3.5)\)
Midpoint of AC, M2 = \(\left(\frac{A_x + C_x}{2}, \frac{A_y + C_y}{2}\right)\) = \(\left(\frac{0+7}{2}, \frac{0+7}{2}\right)\) = \((3.5, 3.5)\)
Find the slopes of the sides AB and AC:
Slope of AB, m1 = \(\frac{B_y - A_y}{B_x - A_x}\) = \(\frac{-7-0}{7-0}\) = \(-1\)
Slope of AC, m2 = \(\frac{C_y - A_y}{C_x - A_x}\) = \(\frac{7-0}{7-0}\) = \(1\)
03
Find the perpendicular bisectors of the two chosen sides
The slope of the line perpendicular to AB is the negative reciprocal of m1: m3 = \(\frac{1}{1}\) = \(1\).
The slope of the line perpendicular to AC is the negative reciprocal of m2: m4 = \(\frac{1}{-1}\) = \(-1\).
Now, we have the slopes and midpoints of the perpendicular bisectors M1R1 and M2R:
Equation of M1R1: y - M1_y = m3 (x - M1_x) ⇒ y - (-3.5) = 1(x - 3.5) ⇒ y + 3.5 = x - 3.5
Equation of M2R: y - M2_y = m4 (x - M2_x) ⇒ y - 3.5 = -1(x - 3.5) ⇒ y - 3.5 = -x + 3.5
04
Calculate the intersection point of the two perpendicular bisectors
To find the circumcentre, we need to solve the two equations obtained in Step 3:
1. y + 3.5 = x - 3.5
2. y - 3.5 = -x + 3.5
Add both equations:
2y = -7
y = -3.5
Substitute the value of y in the first equation:
-3.5 + 3.5 = x - 3.5
0 = x - 3.5
x = 3.5
Hence, the circumcentre is (3.5, -3.5), which is not among the given options.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Triangle Vertices
Understanding triangle vertices is crucial when identifying or analyzing the shape of a triangle. A vertex is simply a point where two or more lines meet, forming a corner. In the given exercise, the triangle is formed by three lines defined by the equations:
- x + y = 0
- x - y = 0
- x - 7 = 0
- The intersection of x + y = 0 and x - y = 0 yields the vertex A(0, 0).
- The intersection of x + y = 0 and x - 7 = 0 gives the vertex B(7, -7).
- The intersection of x - y = 0 and x - 7 = 0 results in the vertex C(7, 7).
Perpendicular Bisectors
Perpendicular bisectors are lines that perpendicularly cross another line segment at its midpoint. This concept is especially useful when finding a triangle's circumcenter, as the circumcenter is the point where the perpendicular bisectors of a triangle's sides intersect.
After identifying the triangle's vertices, we move to the calculation of its perpendicular bisectors.
For a line segment like AB or AC, the slope of the perpendicular bisector is the negative reciprocal of the original slope. For example:
After identifying the triangle's vertices, we move to the calculation of its perpendicular bisectors.
For a line segment like AB or AC, the slope of the perpendicular bisector is the negative reciprocal of the original slope. For example:
- The slope of AB is -1; thus, the perpendicular slope becomes 1.
- The slope of AC is 1; thus, its perpendicular slope becomes -1.
Midpoints of Sides
Midpoints play a vital role in understanding the properties of triangles, particularly when looking for the circumcenter. A midpoint is the point exactly halfway along a side of the triangle. To find it, you calculate the average of the coordinates of the endpoints.
In our example, we need these midpoints to determine the bisectors:
In our example, we need these midpoints to determine the bisectors:
- The midpoint of side AB is calculated as follows: \( \left( \frac{0+7}{2}, \frac{0-7}{2} \right) \), resulting in (3.5, -3.5).
- Similarly, the midpoint of side AC is: \( \left( \frac{0+7}{2}, \frac{0+7}{2} \right) \), which is (3.5, 3.5).
Slopes of Lines
The slope of a line is a measure of its steepness and direction. In the context of triangles, understanding line slopes is essential for finding perpendicular bisectors. Slopes help determine how lines relate to one another:
- Lines with slopes that are negative reciprocals are perpendicular to each other.
- For side AB, with coordinates (0,0) and (7,-7), the slope is calculated by \( \frac{-7}{7} = -1 \).
- For side AC with coordinates (0,0) and (7,7), the slope is \( \frac{7}{7} = 1 \).