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    Express Cos(40)Cos(30) + Sin(40)Sin(30) in the form Sin(X)

    I have tried putting into the form Cos(A+B) then using the cos+sin=1 identity to work it out but i get no where near the answer. I know it might seem an easy question but im not sure how to go about it.

    Thanks for any help
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    The expression is equal to cos(40-30)=cos(10)=sin(80)
    Remember that cos(90-θ)=sinθ.
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    (Original post by ABC1123)
    Express Cos(40)Cos(30) + Sin(40)Sin(30) in the form Sin(X)

    I have tried putting into the form Cos(A+B) then using the cos+sin=1 identity to work it out but i get no where near the answer. I know it might seem an easy question but im not sure how to go about it.

    Thanks for any help
    http://www.thestudentroom.co.uk/forumdisplay.php?f=38
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    Sin(a) = cos(90-a)
    Cos(a) = sin(90-a)

    (Im waiting for that cheeky rep)
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    (Original post by B_9710)
    The expression is equal to cos(40-30)=cos(10)=sin(80)
    Remember that cos(90-θ)=sinθ.
    Thanks
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    (Original post by GeologyMaths)
    Sin(a) = cos(90-a)
    Cos(a) = sin(90-a)

    (Im waiting for that cheeky rep)
    Thanks, and you sure do get the rep!
 
 
 
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