# 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
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
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
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