Asked by Reston
                Find the exact value of cos(u+v) given that sine u=4/5 with u in quadrant II and sine v = -12/13 with v in quadrant IV.
Not sure how to even begin.
            
            
        Not sure how to even begin.
Answers
                    Answered by
            Steve
            
    first, draw your triangles in standard position. Then it is easy to see that
cos(u) = -3/5
cos(v) = 5/13
Then recall your sum formula:
cos(u+v) = cosu cosv - sinu sinv
now just plug in your numbers.
    
cos(u) = -3/5
cos(v) = 5/13
Then recall your sum formula:
cos(u+v) = cosu cosv - sinu sinv
now just plug in your numbers.
                    Answered by
            Bosnian
            
    sin u = 4 / 5
cos u = sqroot ( 1 - sin ^ 2 u )
cos u = sqroot [ 1 - ( 4 / 5 ) ^ 2 ]
cos u = sqroot ( 1 - 16 / 25 )
cos u = sqroot ( 25 / 25 - 16 / 25 )
cos u = sqroot ( 9 / 25 )
cos u = ± 3 / 5
In Quadrant II cos is negative so:
cos u = - 3 / 5
sin v = - 12 /13
cos v = sqroot ( 1 - sin ^ 2 v )
cos v = sqroot [ 1 - ( - 12 / 13 ) ^ 2 ]
cos v = sqroot ( 1 - 144 / 169 )
cos v = sqroot ( 169 / 169 - 144 / 169 )
cos v = sqroot ( 25 / 169 )
cos v = ± 5 / 13
In Quadrant IV cos is postive so:
cos v = 5 / 13
sin u = 4 / 5, cos u = - 3 / 5, sin v = - 12 /13, cos v = 5 / 13,
cos ( u + v ) = cos u * cos v - sin u * sin v
cos ( u + v ) = ( - 3 / 5 ) * ( 5 / 13 ) - ( 4 / 5 ) * ( - 12 / 13 ) =
- 3 / 13 + 48 / 65 =
- 3 * 5 / ( 13 * 5 ) + 48 / 65 =
- 15 / 65 + 48 / 65 = 33 / 65
cos ( u + v ) = 33 / 65
    
cos u = sqroot ( 1 - sin ^ 2 u )
cos u = sqroot [ 1 - ( 4 / 5 ) ^ 2 ]
cos u = sqroot ( 1 - 16 / 25 )
cos u = sqroot ( 25 / 25 - 16 / 25 )
cos u = sqroot ( 9 / 25 )
cos u = ± 3 / 5
In Quadrant II cos is negative so:
cos u = - 3 / 5
sin v = - 12 /13
cos v = sqroot ( 1 - sin ^ 2 v )
cos v = sqroot [ 1 - ( - 12 / 13 ) ^ 2 ]
cos v = sqroot ( 1 - 144 / 169 )
cos v = sqroot ( 169 / 169 - 144 / 169 )
cos v = sqroot ( 25 / 169 )
cos v = ± 5 / 13
In Quadrant IV cos is postive so:
cos v = 5 / 13
sin u = 4 / 5, cos u = - 3 / 5, sin v = - 12 /13, cos v = 5 / 13,
cos ( u + v ) = cos u * cos v - sin u * sin v
cos ( u + v ) = ( - 3 / 5 ) * ( 5 / 13 ) - ( 4 / 5 ) * ( - 12 / 13 ) =
- 3 / 13 + 48 / 65 =
- 3 * 5 / ( 13 * 5 ) + 48 / 65 =
- 15 / 65 + 48 / 65 = 33 / 65
cos ( u + v ) = 33 / 65
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