# Compound Angles?

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## Homework Statement

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.

## Homework Equations

cos(a+b)=cosacosb+sinasinb

## The Attempt at a Solution

I don't know what I'm doing

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed

## Homework Statement

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.

## Homework Equations

cos(a+b)=cosacosb+sinasinb

## The Attempt at a Solution

I don't know what I'm doing

Check your formula in (2.) carefully.

Check your formula in (2.) carefully.
cos(a+b)=cosacosb-sinasinb

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed
cos(a+b)=cosacosb-sinasinb

OK, so what is preventing you from applying that?

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

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed
They want an exact value, not a decimal answer. I don't know what the exact value is for sin2/5 or tan3/4

Why do you think you need to know those values? Read the question again, carefully.

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

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed
Because when I plugged the numbers in and solved for an exact value, I got 12.32/5 and it was wrong apparantly

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.

Mark44
Mentor
They want an exact value, not a decimal answer. I don't know what the exact value is for sin2/5 or tan3/4
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).

Think about what sine represents in terms of sides of triangle.