cos(3pi/10)cos(pi/5)-sin(3pi/10)sin(pi/5)

factor cos and sin and get common denominators and combine:

cos(5pi/10)-sin(5pi/10)

cosine(90 degrees)-sin(90 degrees)

0-1

answer is -1? Am I correct?

Wait I see the formula for this is : cos(a+b)

so then I do cos(3pi/10+pi/5)

cos(5pi/10)

and that =0? Is that how I do it?

factor cos and sin and get common denominators and combine:

cos(5pi/10)-sin(5pi/10)

cosine(90 degrees)-sin(90 degrees)

0-1

answer is -1? Am I correct?

Wait I see the formula for this is : cos(a+b)

so then I do cos(3pi/10+pi/5)

cos(5pi/10)

and that =0? Is that how I do it?

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