Posted by Anonymous on Wednesday, May 20, 2009 at 8:28pm.
How can I evaluate the expression below without using calculator?
cos15*2sin15
Thank you!

Mathematics  Emileee, Wednesday, May 20, 2009 at 9:41pm
you can use the sum and difference rules
cos15 = cos (4530) = cos45*cos30 + sin45*sin30
sin15 = sin (4530) = sin45*cos30  cos45*sin30
45 and 30 are the common trig values to know

Mathematics  Anonymous, Wednesday, May 20, 2009 at 10:00pm
I got sqrt6/4  sqrt2/4 for sin15 and sqrt6/4 + sqrt2/4 for cos15. Is it right?
And the answer would be 6radical3/4???
Thank you!

Mathematics  Anonymous, Wednesday, May 20, 2009 at 10:07pm
Also, can you give me the exact formulas for the sum and difference rules (both sin and cos) without substituting any angles there? Thanks.

Mathematics  Reiny, Wednesday, May 20, 2009 at 10:45pm
OR
cos15*2sin15
= 2(sin15)(cos15)
= sin 30
= 1/2

Mathematics  Anonymous, Wednesday, May 20, 2009 at 11:10pm
How did you do this, Reiny? I don't understand. How does 2(sin15)(cos15) equal sin30? Please help!
Thanks.

Mathematics  Reiny, Wednesday, May 20, 2009 at 11:14pm
the "halfangle" formula
sin 2A = 2(sinA)(cosA)
in this case A = 15ยบ
by the way, you can check on a calculator
your (cos15)(2sin15) = .5 or 1/2
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