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Question

I n a triangle A B C comma a left parenthesis c o s squared invisible function application B plus c o s squared invisible function application C right parenthesis plus c o s invisible function application A left parenthesis c c o s invisible function application C plus b c o s invisible function application B right parenthesis equals

  1. a
  2. b
  3. c
  4. a plus b plus c

hintHint:

First we will rearrange the terms then we will use the trigonometric formula a equals space b cos open parentheses C close parentheses plus c cos open parentheses B close parentheses
b space equals space c cos open parentheses A close parentheses plus a cos open parentheses C close parentheses
c equals space a cos open parentheses B close parentheses plus b cos open parentheses A close parentheses .

The correct answer is: a plus b plus c


    Step1:  Rewriting the expression
    a cos squared open parentheses B close parentheses plus a cos squared open parentheses C close parentheses plus space c cos open parentheses A close parentheses cos open parentheses C close parentheses plus b cos open parentheses A close parentheses cos open parentheses B close parentheses
    => a cos squared open parentheses B close parentheses plus b cos open parentheses A close parentheses cos open parentheses B close parentheses plus a cos squared open parentheses C close parentheses plus space c cos open parentheses A close parentheses cos open parentheses C close parentheses
    Step2:  Using the trigonometric formula
    a equals space b cos open parentheses C close parentheses plus c cos open parentheses B close parentheses
b space equals space c cos open parentheses A close parentheses plus a cos open parentheses C close parentheses
c equals space a cos open parentheses B close parentheses plus b cos open parentheses A close parentheses
    => cos open parentheses B close parentheses left parenthesis a cos open parentheses B close parentheses plus b cos open parentheses A close parentheses right parenthesis plus cos open parentheses C close parentheses left parenthesis a cos open parentheses C close parentheses plus space c cos open parentheses A close parentheses right parenthesis
    => c cos open parentheses B close parentheses plus b cos open parentheses C close parentheses
    Step3: Again using the trigonometric formula
    => c cos open parentheses B close parentheses plus b cos open parentheses C close parentheses
    =>a

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