Asked by mysterychicken

Can someone please check my answers!
2. Find value of cos(255degrees)cos(105degrees)
root3 - 2 / 4

3. cos(pi/12) - cos(5pi/12)
Is it root3/4?

4. Use the appropriate sum-to-product formula to rewrite the expression sin6x - sin9x
I don't really understand how to do these, but I got -2sin(3x/2)cos(15x/2)..

5. same type of question: rewrite the expression cos4x - cos3x
Is it cosx??
Answered by bobpursley
on all these, they are the same thing. I am not certain how you see 4,5 as different.

look here: http://www.sosmath.com/trig/Trig5/trig5/trig5.html Print that out. Notice the sum to product formulas. If you were my student, I would have you write each formula on a flash card until you memorized it.

work on using those formulas on that page.
Answered by mysterychicken
Okay, I understand 4 and 5. But what about 2 and 3?
Answered by mysterychicken
Nevermind, I get it
Answered by Reiny
check my solutions for #1, and #2
http://www.jiskha.com/display.cgi?id=1370293791
Answered by bobpursley
3. CosA-cosB= -2(sin((a+b)/2)*sin((a-b)/2)
THEN
COS(pi/12) - cos(5pi/12)
= -2(sin( SUM/2)*sin(DIFF/2)
= -2(SIN 6pi/24 *SIN(-4pi/24))
= -2 SIN pi/4 * SIN -pi/6)
= 2SIN (45)* SIN30
= SQRT2*1/2=.707

2/ USE THE FORMULA

COSa*COSb= 1/2 (COS(A-B)+COS(a+B) )
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