- #1

Maatttt0

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[STRIKE]

Given that cos C = 12/13 where C is a reflex and sin D = 3/5 where D is acute, find the exact value of cos ( C + D ).

I used the Addition Formulae: cos(A+B) = cosAcosB-sinAsinB

cos(12/13)cos(2/5) - sin(1/13)sin(3/5) = 0.999831...

As it says "exact value" I assume my long decimal answer is incorrect. I have inputed it all straight into my calculator. Help me please [/STRIKE]

## Homework Statement

Given that cos C = 12/13 where C is a reflex and sin D = 3/5 where D is acute, find the exact value of cos ( C + D ).

## Homework Equations

## The Attempt at a Solution

I used the Addition Formulae: cos(A+B) = cosAcosB-sinAsinB

cos(12/13)cos(2/5) - sin(1/13)sin(3/5) = 0.999831...

As it says "exact value" I assume my long decimal answer is incorrect. I have inputed it all straight into my calculator. Help me please [/STRIKE]

**Sorry - I've worked out my silly mistake. Thank you for looking**
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