Right Triangle Trigonometry

Transcription

1 Section 6.4 OBJECTIVE : Right Triangle Trigonometry Understanding the Right Triangle Definitions of the Trigonometric Functions otenuse osite side otenuse acent side acent side osite side We will be concerned about the lengths of each of these three sides. For simplicity, we will abbreviate the length of the otenuse as. The length of the osite side will be referred to as and the length of the acent side will be denoted as. Choosing side lengths two at a time, we can create a total of six ratios. These six ratios are,,,,, and. The value of each of these six ratios depends on the measure of the acute angle. Thus, the six ratios are functions of the variable and are called the trigonometric functions of the acute angle. For convenience, the six ratios have been given names. Historically, these six trigonometric functions have been named sine of theta, cosine of theta, tangent of theta, cosecant of theta, secant of theta, and cotangent of theta. The six functions are abbreviated as sin, cos, tan, csc, sec,and cot. The Right Triangle Definition of the Trigonometric Functions Given a right triangle with acute angle and side lengths of,, and, the six trigonometric functions of angle are defined as follows: sin csc cos sec tan cot

2 It is extremely important to memorize the six trigonometric functions. To memorize the ratios for sin, cos,and tan, you could use the following silly phrase: Some Old Horse Caught Another Horse Taking Oats Away sin cos tan You might also use the acronym SOHCAHTOA pronounced So-Kah-Toe-Ah to help you memorize these functions. Once you have memorized the ratios for sin, cos,and tan, you can easily obtain the ratios of csc, sec,and cot since the ratios for csc, sec,and cot are simply the reciprocals of the ratios for sin, cos,and tan respectively. The value of the six trigonometric functions for a specific acute angle will be exactly the same regardless of the size of the triangle (similar triangles) If is an acute angle of a right triangle and if, then find the values of the remaining five trigonometric functions for angle. OBJECTIVE 2: Using the Special Right Triangles Check your knowledge: Draw a,, 45,45,90 triangle with leg length ,60,90 Draw a,, triangle with length of the shortest side. Confirm values in Table. NOTE: These values do not have to be rationalized.

3 6.4.8 Use special right triangles to evaluate the expression. OBJECTIVE 3: Understanding the fundamental Trigonometric Identities Trigonometric identities are equalities involving trigonometric expressions that hold true for any angle θ for which all expressions are defined. The Quotient Identities sin tan cos cos cot sin Use the Right Triangle Definition of the Trigonometric Functions in Objective to prove one of the Quotient Identities. The Reciprocal Identities sin csc csc sin cos sec sec cos tan cot cot tan Use the Right Triangle Definition of the Trigonometric Functions in Objective to prove one of the Reciprocal Identities.

4 The Pythagorean Identities sin cos tan sec cot csc Use the Right Triangle Definition of the Trigonometric Functions in Objective to prove one of the Pythagorean Identities Use identities to find the exact value of the trigonometric expression. Assume is an acute angle. OBJECTIVE 4: Understanding Cofunctions Prove the measures of the acute angles are as shown to the right. Cofunction Identities sin cos 2 tan cot 2 sec csc 2 cos sin 2 cot tan 2 csc sec 2 If angle is given in degrees, replace 2 with Rewrite the expression as one of the six trigonometric functions of acute angle.

5 OBJECTIVE 5: Evaluating Trigonometric Functions Using a Calculator Look at a scientific calculator. It has a sin key, a cos key, and a tan key. It can also be in degree mode or in radian mode. Make sure it is in the correct mode for the values you enter. Reciprocal identities are required to evaluate,, and. Steps to evaluate,, and.. Make sure your calculator is in the correct mode. 2. Rewrite the function using its reciprocal identity and evaluate using order of operations Use a calculator to evaluate the trigonometric expression. Round your answer to 4 decimal places. OBJECTIVE 6: Applications of Right Triangle Trigonometry Exam questions will use special triangles or will ask students to set up a problem without evaluating to avoid the necessity of calculators The angle of elevation to the top of a flagpole is radians from a point on the ground feet away from its base. Find the height of the flagpole. Round to 2 decimal places A mine shaft with a circular entrance has been carved into the side of a mountain. From a distance of feet from the base of the mountain, the angle of elevation to the bottom of the circular opening is. The angle of elevation to the top of the opening is. Determine the diameter of the circular entrance.