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A 0.2 kg ball is thrown with an initial velocity of 20 m/s at an angle of 30 degrees with respect to the horizontal. What is the kinetic energy of the ball at the top of it's path?

I don't understand why in order to get the answer, you have to use the horizontal velocity componenent, Vx. Shouldn't you just use the velocity as given, and plug it into .5mv^2? Thanks