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How do you verify the identity ##sin(pi/2 + x) = cosx##?

    for the “true” proof you need to use matrice, but this is acceptable :

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

    ##sin(pi/2+x) = sin(pi/2)*cos(x)+cos(pi/2)*sin(x)##

    ##sin(pi/2) = 1## ##cos(pi/2) = 0 ##

    So we have :

    ##sin(pi/2+x) = cos(x)##