Posted by Jossie on Thursday, March 21, 2013 at 4:16am.
evaluate each of the following...sin 75-cos 105-sin 15+cos 15
mathematics - Reiny, Thursday, March 21, 2013 at 9:49am
I assume you want "exact" answers for these, the usual kind of request for these types of questions.
All your given angles are combinations of the standard angles of 30, 45, 60 etc
e.g. 75 = 45+30
15 = 45-30
105 = 60+45
You should also have memorized or have quick access to the trig ratios of these
e.g. sin 45° = √2/2 , cos 60° = 1/2 , etc
evaluate each one, the do the arithmetic
sin75 = sin(45+30)
= sin45cos30 + cos45sin30
= √2/2(√3/2) + √2/2(1/2)
= √6/4 + √2/4 = (√6 + √2)/4
carefully do the others, easy to make errors with those radicals
mathematics - Reiny, Thursday, March 21, 2013 at 9:54am
You could also have done some preliminaray simplification using the complementary angle properties
cos15° = sin75°
cos105 = sin(-15) = -sin15
so sin 75-cos 105-sin 15+cos 15
sin75 - (-sin15) + sin15 + sin75
= 2sin75 + 2sin15
Now you only have to do two of the calculations
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