Question #9f971 Trigonometry Right Triangles Trigonometric Functions of Any Angle 1 Answer Konstantinos Michailidis Feb 28, 2016 It is #-sqrt3/2# Explanation: It is #cos (7pi/4)cos(7pi/12)+sin(7pi/4)sin(7pi/12)=cos(7pi/4-7pi/12)= cos(7*pi/6)=cos(pi+pi/6)=-cos(pi/6)=-sqrt3/2# Note We used the identity #cos(A-B)=cosA*cosB+sinA*sinB# Answer link Related questions How do you find the trigonometric functions of any angle? What is the reference angle? How do you use the ordered pairs on a unit circle to evaluate a trigonometric function of any angle? What is the reference angle for #140^\circ#? How do you find the value of #cot 300^@#? What is the value of #sin -45^@#? How do you find the trigonometric functions of values that are greater than #360^@#? How do you use the reference angles to find #sin210cos330-tan 135#? How do you know if #sin 30 = sin 150#? How do you show that #(costheta)(sectheta) = 1# if #theta=pi/4#? See all questions in Trigonometric Functions of Any Angle Impact of this question 1088 views around the world You can reuse this answer Creative Commons License