美国高考SAT专题分析——Triangles
时间：2008-10-17点击：
Three Sides, Four Fundamental Properties
Every triangle, no matter how special, follows four main rules.
1. Sum of the Interior Angles
If you were trapped on a deserted island with tons of SAT questions about triangles, this is the one rule you’d need to know:
The sum of the interior angles of a triangle is 180°.
If you know the measures of two of a triangle’s angles, you’ll always be able to find the third by subtracting the sum of the first two from 180.
2. Measure of an Exterior Angle
The exterior angle of a triangle is always supplementary to the interior angle with which it shares a vertex and equal to the sum of the measures of the remote interior angles. An exterior angle of a triangle is the angle formed by extending one of the sides of the triangle past a vertex. In the image below, d is the exterior angle.
Since d and c together form a straight angle, they are supplementary: 
. According to the first rule of triangles, the three angles of a triangle always add up to 
, so 
. Since 
and 
, d must equal a + b.
. According to the first rule of triangles, the three angles of a triangle always add up to , so . Since and , d must equal a + b. 3. Triangle Inequality Rule
If triangles got together to write a declaration of independence, they’d have a tough time, since one of their defining rules would be this:
The length of any side of a triangle will always be less than the sum of the lengths of the other two sides and greater than the difference of the lengths of the other two sides.
There you have it: Triangles are unequal by definition.
Take a look at the figure below:
The triangle inequality rule says that c – b < a < c + b. The exact length of side a depends on the measure of the angle created by sides b and c. Witness this triangle:
Using the triangle inequality rule, you can tell that 9 – 4 < x < 9 + 4, or 5 < x < 13. The exact value of x depends on the measure of the angle opposite side x. If this angle is large (close to 
) then x will be large (close to 13). If this angle is small (close to 
), then x will be small (close to 5).
) then x will be large (close to 13). If this angle is small (close to ), then x will be small (close to 5). The triangle inequality rule means that if you know the length of two sides of any triangle, you will always know the range of possible side lengths for the third side. On some SAT triangle questions, that’s all you’ll need.
4. Proportionality of Triangles
Here’s the final fundamental triangle property. This one explains the relationships between the angles of a triangle and the lengths of the triangle’s sides.
In every triangle, the longest side is opposite the largest angle and the shortest side is opposite the smallest angle.
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In this figure, side a is clearly the longest side and 
is the largest angle. Meanwhile, side c is the shortest side and 
is the smallest angle. So c < b < a and C < B < A. This proportionality of side lengths and angle measures holds true for all triangles.
is the largest angle. Meanwhile, side c is the shortest side and is the smallest angle. So c < b < a and C < B < A. This proportionality of side lengths and angle measures holds true for all triangles. See if you can use this rule to solve the question below:
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According to the proportionality of triangles rule, the longest side of a triangle is opposite the largest angle. Likewise, the shortest side of a triangle is opposite the smallest angle. The largest angle in triangle ABC is 
, which is opposite the side of length 8. The smallest angle in triangle ABC is 
, which is opposite the side of length 6. This means that the third side, of length x, measures between 6 and 8 units in length. The only choice that fits the criteria is 7. C is the correct answer.
, which is opposite the side of length 8. The smallest angle in triangle ABC is , which is opposite the side of length 6. This means that the third side, of length x, measures between 6 and 8 units in length. The only choice that fits the criteria is 7. C is the correct answer. Special Triangles
Special triangles are “special” not because they get to follow fewer rules than other triangles but because they get to follow more. Each type of special triangle has its own special name: isosceles, equilateral, and right. Knowing the properties of each will help you tremendously, humongously, a lot, on the SAT.
But first we have to take a second to explain the markings we use to describe the properties of special triangles. The little arcs and ticks drawn in the figure below show that this triangle has two sides of equal length and three equal angle pairs. The sides that each have one tick through them are equal, as are the sides that each have two ticks through them. The angles with one little arc are equal to each other, the angles with two little arcs are equal to each other, and the angles with three little arcs are all equal to each other.
Isosceles Triangles
In ancient Greece, Isosceles was the god of triangles. His legs were of perfectly equal length and formed two opposing congruent angles when he stood up straight. Isosceles triangles share many of the same properties, naturally. An isosceles triangle has two sides of equal length, and those two sides are opposite congruent angles. These equal angles are usually called as base angles. In the isosceles triangle below, side a = b and 
:
:
If you know the value of one of the base angles in an isosceles triangle, you can figure out all the angles. Let’s say you’ve got an isosceles triangle with a base angle of 35º. Since you know isosceles triangles have two congruent base angles by definition, you know that the other base angle is also 35º. All three angles in a triangle must always add up to 180º, right? Correct. That means you can also figure out the value of the third angle: 180º – 35º – 35º = 110º.
