GMAT® Official Guide 2019 Quantitative Review
The commonly used phrase "area of a triangle" (or any other plane figure) is used to mean the area of the
region enclosed by that figure.- Triangles
There are several special types of triangles with important propeirties. But one property that all triangles
share is that the sum of the lengths of any two of the sides is greater than the length of the third side, as
illustrated below.
z
x + y > z, x + z > y, and y + z > xAn equilateral triangle has all sides of equal length. All angles of an equilateral triangle have equal
measure. An isosceles triangle has at least two sides of the same length. If two sides of a triangle have the
same length, then the two angles opposite those sides have the same measure. Conversely, if two angles
of a triangle have the same measure, then the sides opposite those angles have the same length. In
isosceles triangle PQR below, x = y since PQ = QR.Q10/\10
p L1 R
A triangle that has a right angle is a right triangle. In a right triangle, the side opposite the right angle is
the hypotenuse, and the other two sides are the legs. An important theorem concerning right triangles is
the Pythagorean theorem, which states: In a right triangle, the square of the length of the hypotenuse is
equal to the sum of the squares of the lengths of the legs.s
In the figure above, l::!.RSTis a right triangle, so (RS)^2 + (RT)^2 = (ST)2. Here, RS= 6 and RT= 8, so
ST= 10 , since 62 + 82 = 36 + 64 = 100 = (ST)^2 and ST= -Jioo. Any triangle in which the lengths of
the sides are in the ratio 3:4:5 is a right triangle. In general, if a, b, and care the lengths of the sides of a
triangle and a^2 + b^2 = c^2 , then the triangle is a right triangle.