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# The pythagorean theorem

The square of the length of the hypotenuse of a right triangle is equal to the sum of the square of the lengths of the legs. In right triangle ABC, AC2 = AB2 + BC2. In right triangle ABC, there is one hypotenuse which opposite the right angle. There are two legs which are two sides of the right angle. hypotenuse2 = Leg12 + Leg22

# Converse of the pythagorean theorem

In a triangle, if the square of the length of a side is equal to the sum of the square of the lengths of other two sides, then the triangle is a right triangle. In the figure above, if AB2 = AC2 + BC2, then the triangle ABC is a right triangle. or if c2 = a2 + b2, then the triangle ABC is a right triangle. Note: in triangle ABC, a is the side BC which opposite the angle A, b is the side AC which opposite the angle B and c is the side AB which opposite the angle C.

# Median throrem of a right triangle

In a right triangle, the length of the median of hypotenuse drawn from the hypotenuse to its opposite angle is one-half the length of the hypotenuse. In the figure above, if triangle ABC is a right triangle, AB is the hypotenuse and D is the midpoint of AB, then CD = (1/2)AB. CD is the median of the hypotenuse.

# Property of the 30o-60o-90o right triangle

In a right triangle, if an angle is 30 degrees, then the length of its opposite side is one-half the length of the hypotenuse. In right triangle ABC, if angle B = 30o, then AC = (1/2)AB.

Note: AB is the hypotenuse of the right triangle ABC. AC is a leg which opposite the angle B.

# Corollary of the 30o-60o-90o right triangle

In a right triangle, if the length of a leg is one-half the length of the hypotenuse, then the angle opposite this leg is 30 degrees. In right triangle ABC, if AC = (1/2)AB, then the angle B = 30o.

Note: AB is the hypotenuse and AC is a leg. The hypotenuse is opposite the right angle.

# The 45o-45o-90o right triangle

In a right triangle, if an angle is 45 degrees, then the length of the hypotenuse is the product of square root 2 and a leg. In right triangle ABC, since angle A = 45o
So angle B = 90o - 45o = 45o
Since angle A = angle B = 45o
So AC = BC (In a triangle, equal angles opposite equal sides.)
In right triangle ABC, AB is the hypotenuse. AC and BC are legs.
AB2 = AC2 + BC2
Since AC = BC, so
AB2 = 2AC2 The length of the hypotenuse is equal to the length of either leg multiplied by square root of 2.
The length of either leg is equal to one-half the length of the hypotenuse multiplied by square root of 2.

# Definition of sine, cosine and tangent<     Example 1

In right triangle ABC, if the angle B = 30o and AB = 6, what is the length of BC? Solution
In the right triangle ABC, since angle B = 30o and angle B opposite AC,
and since angle C = 90o and angle C opposite AB, so AB is the hypotenuse,
so AC = (1/2)AB = (1/2) × 6 = 3
(In a right triangle, the length of the leg opposite 30o is one-half the length of the hypotenuse.)
AB2 = AC2 + BC2 (the pythagorean theorem)
BC2 = AB2 - AC2 = 62 - 32 = 36 - 9 = 27 = 32 × 3
BC = 3(square root of 3)

Example 2

In the right triangle ABC, if the angle B = 45o and BC = 5. Find the length of AB. Solution
In right triangle ABC, since angle B = 45o and angle C = 90o,
so angle A = 90o - 45o = 45o
so AC = BC = 5 (In a triangle, equal angles opposite equal sides)
then AB2 = AC2 + BC2 = 2AC2 = 2 × 52
so AB = 5(square root of 2)

Example 3

In the triangle ABC, angle B = 45o, angle C = 30o and AD is perpendicular to BC at D. AD = 2, find the length of BC. (point D lies on BC) Solution
Since AD is perpendicular to BC at point D (Given), so
triangle ABD and triangle ACD are right triangles.
In right triangle ACD,
since angle C = 30o and AD = 2,
so AC = 2 AD = 2 × 2 = 4
CD2 = AC2 - AD2 = 42 - 22 = 16 - 4 = 12 = 3 × 22
CD = 2(square root of 3)
In right triangle ABD,
since angle B = 45o, so angle BAD = 45o
so BD = AD = 2 (triangle ABD is an isosceles right triangle)
BC = BD + CD = 2 + 2(square root of 3) = 2(1 + square root of 3)