SOLUTION: Use the sum/difference identities to simplify the expression. Do not use a calculator. {{{cos (7pi/18) cos (2pi/9) + sin (7pi/18) sin (2pi/9) }}} Answer : {{{ cos (pi/6) }

Algebra ->  Trigonometry-basics -> SOLUTION: Use the sum/difference identities to simplify the expression. Do not use a calculator. {{{cos (7pi/18) cos (2pi/9) + sin (7pi/18) sin (2pi/9) }}} Answer : {{{ cos (pi/6) }      Log On


   

Question 932383: Use the sum/difference identities to simplify the
expression. Do not use a calculator.
cos+%287pi%2F18%29+cos+%282pi%2F9%29+%2B+sin+%287pi%2F18%29+sin+%282pi%2F9%29+

Answer : +cos+%28pi%2F6%29+

Could you explain the steps to get to the answer? Thank you
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
cos+%28x%29+cos+%28y%29+%2B+sin+%28x%29+sin+%28y%29+=+cos%28x-y%29+ (one of the many trig identities)


Plug in x+=+7pi%2F18 and y+=+2pi%2F9













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