The particle is moving in a 3-D coordinate system.
Displacement in x-direction is given as x = a cos

t.
Differentiate both sides wrt t to get component of velocity in x-direction.
x = dx/dt = a(–sin

t) .

= – a

sin

t [

a is constant]
Similarly, y = a sin

t
y = dy/dt = a

cos

t
Speed of the particle v is the magnitude of the vector sum of the velocity components.
x,y and z axes are mutually perpendicular.
Thus, the speed of the particle is

.