Verified answer

you should know that: cosx = sen (90° - x)

cos ( x -90°) = sen( 90° -x+90°) = sen ( 180° -x) = senx

y= - senx,

here's your graph : http://www.wolframalpha.com/input/?i=y%3D+-+senx+

Download Graph 4.4 from www.padowan.dk for free.

On "Insert function", type -cos(x-(pi/2)), then "OK".