Perpendicular and Angle Bisectors

Imagine a line that cuts a segment exactly in half and forms right angles with it - that's a perpendicular bisector. It's super useful in geometry because of two key theorems:

Theorem 5-1 states that if a point is on the perpendicular bisector of a segment, then it's equidistant (the same distance) from both endpoints of the segment. The reverse is also true in Theorem 5-2 - if a point is equidistant from both endpoints, it must be on the perpendicular bisector.

Angle bisectors work in a similar way. Theorem 5-3 tells us that if a point is on an angle bisector, it's equidistant from both sides of the angle. And according to Theorem 5-4, if a point is equidistant from the sides of an angle, it must be on the angle bisector.

💡 Think of perpendicular bisectors and angle bisectors as "fairness lines" - any point on them is equally distant from both parts they're separating!

To solve these problems, look for equal distances as clues about where points lie, or use the distance properties to find unknown values in equations.

Bisectors in Triangles

Did you know triangles have special circles connected to them? A circumscribed circle passes through all three vertices of a triangle, while an inscribed circle touches each side of a triangle exactly once.

These circles have special centers. The circumcenter is where all three perpendicular bisectors of a triangle's sides meet. What makes this point amazing is that it's exactly the same distance from all three vertices - that's why it works as the center of the circumscribed circle!

The incenter is where all three angle bisectors meet. This point is equidistant from all three sides of the triangle, making it perfect as the center of the inscribed circle.

🔍 When lines or rays meet at a single point, we call them concurrent, and that meeting point is called a point of concurrency.

Remember that the radius is half of the diameter, which can help you solve problems. For example, if 2x-5 = x+1, then x = 6, making the radius 7 and the diameter 14.

Medians and Altitudes

Every triangle has special lines that tell us important things about its shape. An altitude is a perpendicular segment from a vertex to the opposite side, while a median connects a vertex to the midpoint of the opposite side.

The centroid is where all three medians of a triangle meet. It divides each median in a special ratio - it's located two-thirds of the distance from any vertex to the midpoint of the opposite side. This means if you travel from a vertex along a median, you'll hit the centroid after going 2/3 of the way.

The orthocenter is where the lines containing the altitudes meet. What's interesting is that its position changes based on the triangle type:

• In a right triangle, it's at the right angle vertex
• In an acute triangle, it's inside the triangle
• In an obtuse triangle, it's outside the triangle

⚡ The centroid actually serves as the triangle's center of gravity or balance point - if you made a triangle out of cardboard, it would balance perfectly on its centroid!

Remembering these locations will help you identify triangle types and solve problems involving these special points.

Finding Centers of Triangles

Finding the centroid of a triangle is pretty straightforward. If you know the coordinates of the three vertices, just calculate the average of the x-coordinates and the average of the y-coordinates. For vertices (5,-11), (3,5), and (7,8), the centroid is at (5,⅔).

Finding the orthocenter takes a bit more work. Here's how to do it:

First, find the slopes of two sides of the triangle using the formula $y₂-y₁$/$x₂-x₁$. Then find the slopes of the perpendicular lines - remember that perpendicular lines have slopes that are negative reciprocals of each other!

Next, write the equations of the altitude lines using point-slope form: y-y₁=m$x-x₁$. Use the vertex opposite to each side as your point. Finally, solve the system of equations where these lines intersect to find the orthocenter.

🔑 A quick check: If you found an orthocenter that's inside your triangle, your triangle should be acute. If it's outside, your triangle is obtuse. And if it's exactly at a vertex, you've got a right triangle!

This process might seem complicated at first, but with practice, you'll get faster at finding these special points.

Inequalities in One Triangle

Triangles have some cool relationships between their sides and angles. Theorems 5-9 and 5-10 tell us that the shortest side of a triangle is always opposite the smallest angle. Similarly, the longest side is opposite the largest angle.

The Triangle Inequality Theorem (5-11) is super important - it states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side. This helps us determine if three lengths can actually form a triangle. For example, sides of 5, 8, and 11 work because:

• 5 + 8 > 11
• 5 + 11 > 8
• 8 + 11 > 5

You can also use this theorem to find the possible range for a third side. If you know two sides are 200m and 300m, then the third side must be between 100m and 500m because:

• Third side > 300 - 200 (difference of the sides)
• Third side < 300 + 200 (sum of the sides)

💡 Whenever you're given a problem about whether certain lengths can form a triangle, immediately check the Triangle Inequality Theorem - it's the quickest way to verify!

These inequalities help us understand what makes triangles work and why certain combinations of sides or angles are impossible.

Inequalities in Two Triangles

When comparing triangles, the Hinge Theorem gives us powerful insights. If two triangles have two pairs of congruent sides, but one angle (the "hinge") is larger than the corresponding angle in the other triangle, then the third side in that triangle will be longer too.

Think of it like opening a door - as the angle gets wider, the distance between the ends increases. If triangles WXY and ABC have WX ≅ AB and WY ≅ AC, but angle W is greater than angle A, then XY must be greater than BC.

The converse is also true - if that third side is longer, then the angle must be larger too. This helps us compare triangles even when we don't have complete information about all sides and angles.

🌟 The Hinge Theorem might seem abstract, but it's actually something you experience in real life! When you open your arms wider (increasing the angle), the distance between your hands increases too.

These theorems let us write inequalities involving variables. For example, problems might ask you to find the range of possible values for x that satisfy certain conditions about sides or angles in triangles.