Kite is ft above the ground.

Kite moves horizontally at a speed of ft/s.

Assume that kite be at the point and string on the ground at the point .

Assume horizontal distance between the kite and ground is .

Length of the string is .

Speed of the kite is ft/s.

Consider .

Find the horizontal distance from kite to the string.

Consider

Substitute in above expression.

Find the relation between the horizontal distance and height:

Consider .

Differentiate on each side with respect to .

Substitute , and in above expression.

Here the negative sign indicates angle is decreasing.

Rate at which angle between the string and the horizontal decreasing is  rad/s.

Rate at which angle between the string and the horizontal decreasing is  rad/s.