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Posted by on Tuesday, March 22, 2011 at 10:03pm.

Write the expression as a trigonometric function of one number.
(cos 20 degrees)(cos 70 degrees)+(sin 20 degrees)(sin 70 degrees)

  • Trigonometry - , Tuesday, March 22, 2011 at 10:06pm

    Have you come across the identity

    cos(A-B) = cosAcoB + sinAsinB ?

    your question fits that pattern perfectly.

    Let me know what you get.

  • Trigonometry - , Tuesday, March 22, 2011 at 10:11pm

    ok I made a mistake in the formula. it's cos(20 degrees)cos(70 degrees)- sin(20 degrees) sin(70 degrees).

    so it's a subraction equation not an addition.

  • Trigonometry - , Tuesday, March 22, 2011 at 10:13pm

    and of course there is the matching formula for

    cos(A+B) = cosAcosB - sinAsinB

    So what do you think?

  • Trigonometry - , Tuesday, March 22, 2011 at 10:15pm

    so it would be cos(20+70)?

  • Trigonometry - , Tuesday, March 22, 2011 at 10:16pm

    cos(20 degrees+70 degrees) i mean

  • Trigonometry - , Tuesday, March 22, 2011 at 10:38pm

    yes, but what is cos(20° + 70°) ??
    = cos 90°
    = 0

    so your
    cos(20 degrees)cos(70 degrees)- sin(20 degrees) sin(70 degrees)
    = 0

  • Trigonometry - , Tuesday, March 22, 2011 at 10:49pm

    Yeah I got the exact value I just needed it in trigonometric function of one number which would be the degrees of cos.

    Thanks!!!

  • Trigonometry - , Friday, April 15, 2011 at 10:12pm

    HELP ME !

    sin50 cos 20 - cos 50 sin 20
    ^find exact value

  • Trigonometry - , Tuesday, July 26, 2011 at 1:32am

    Pls tell me how to find out sin70.Pls

  • Trigonometry - , Tuesday, June 10, 2014 at 7:45pm

    How are the sun 70 degrees and cos 20 seg related

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