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#1

$Old$ November 9th, 2009, 07:20 AM

Oasis1993 $Oasis1993 is offline$

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Given that sin A= 3/5 and cosB= 12/13 where A is obtuse and B is acute, find the exact values of cos(A+B) and cot(A-B)

I am really having trouble understanding this question, i dont even know how to start..?

Id appreciate the help.
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#2

$Old$ November 9th, 2009, 07:36 AM

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If A is obtuse then $\cos A<0\Rightarrow\cos A=-\sqrt{1-\sin^2A}$ .

If B is acute then $\sin B>0\Rightarrow\sin B=\sqrt{1-\cos^2B}$ .

Then use the identities $\cos(A+B)=\cos A\cos B-\sin A\sin B$

and $\cot(A-B)=\frac{\cot A\cot B+1}{\cot B-\cot A}$

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$Old$ November 9th, 2009, 08:37 AM

Oasis1993 $Oasis1993 is offline$

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How do we find the values of Sin B and Cos A?

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$Old$ November 9th, 2009, 09:08 AM

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Hello Oasis1993

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Originally Posted by Oasis1993 $View Post$

How do we find the values of Sin B and Cos A?

Have you really looked at red_dog's reply?

Grandad

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