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How you prove that ##sin(90+30)=cos30##?

    You need the identity

    ##sin(a+b)=sin(a)cos(b)+cos(a)sin(b)##

    Just plugging in the numbers

    ##sin(90+30)=sin(90)cos(30)+cos(90)sin(30)=cos(30)##

    Since ##sin(90)=1## and ##cos(90)=0##

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