Trig identities....help : |

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frauleinedoctor

Junior Member
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Jan 9, 2009
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Having trouble with 2sin2x - sqr root (2) = 0

Do I square everything first or move the square root 2 over to the other side.....Sort of confused.
 
But what would you do after you get it to that point? How would you get rid of the 2 in sin2x in order to reduce it down to just sinx?
 
Ok that problems fine. But heres another issue Im having:

Writing this expression as sine, cosine or tangent:

cos25degrees x cos15degrees - sin25degrees x sin15degrees


????
 


In the future, please start a NEW TOPIC for each new exercise.

Do you have a textbook? Perhaps, there is a list of the basic identities on one of the inside covers.

If we let x = 25 degrees and y = 15 degrees, then the given expression is as follows.

cos(x) * cos(y) - sin(x) * sin(y)

Does this remind you of any particular identity?

 


In this case, I hope you're not using a math text.

Here are the addition and subtraction formulas.

sin(x + y) = sin(x) * cos(y) + cos(x) * sin(y)

sin(x - y) = sin(x) * cos(y) - cos(x) * sin(y)

cos(x + y) = cos(x) * cos(y) - sin(x) * sin(y)

cos(x - y) = cos(x) * cos(y) + sin(x) * sin(y)

Now your second exercise is easy, right?

The basic identities are listed HERE, along with a bunch of other neat stuff.

 
frauleinedoctor said:
… I plug in my problem … But what does that do?
Click to expand...


It allows you to rewrite the given expression as the cosine of a single angle.

That's what the instructions that you posted for this exercise tell you to do.

 


Yes.

Look for them using the link that I provided (see "5. Angle sum and difference identities" in the table of contents).

 
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