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Compound Angles?

  1. Jan 25, 2015 #1
    • Member warned about posting with no effort shown
    1. The problem statement, all variables and given/known data
    Angle a is located in the second quadrant where sin a=2/5 and angle b is located in the third quadrant where tan b=3/4. Determine an exact value for cos(a+b) and where it is located.

    2. Relevant equations
    cos(a+b)=cosacosb+sinasinb

    3. The attempt at a solution
    I don't know what I'm doing
     
  2. jcsd
  3. Jan 25, 2015 #2

    Ray Vickson

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    Check your formula in (2.) carefully.
     
  4. Jan 25, 2015 #3
    cos(a+b)=cosacosb-sinasinb
     
  5. Jan 25, 2015 #4

    Ray Vickson

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    OK, so what is preventing you from applying that?
     
  6. Jan 25, 2015 #5
    They want an exact value, not a decimal answer. I don't know what the exact value is for sin2/5 or tan3/4
     
  7. Jan 25, 2015 #6

    Ray Vickson

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    Why do you think you need to know those values? Read the question again, carefully.
     
  8. Jan 25, 2015 #7
  9. Jan 25, 2015 #8
    Because when I plugged the numbers in and solved for an exact value, I got 12.32/5 and it was wrong apparantly
     
  10. Jan 25, 2015 #9

    Ray Vickson

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    Let me repeat: read the question carefully, and show your work. We cannot tell where you are going wrong if we cannot see the details of what you have done.
     
  11. Jan 25, 2015 #10

    Mark44

    Staff: Mentor

    These value are not correct. It's not sin(2/5) -- it's sin(a) = 2/5, and similar for angle b. You are given that sin(a) = 2/5. Using right triangle trig, you should be able to find the exact value for cos(a) (don't use a calculator), and similarly for sin(b) and cos(b).
     
  12. Jan 25, 2015 #11
    Think about what sine represents in terms of sides of triangle.
     
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