Hello, anonymous_maths!
A spotlight is on the ground 20m away from a wall.
A 2m tall person is walking towards the wall at a rate of 1.5m/s.
How fast is the height of the shadow changing when the person is 8m from the wall?
Is the shadow increasing or decreasing in height at this time? Code:

S o
:  *
:  *
:  * P
h  o
:   *
:  2 *
:   *
T ooo L
: x Q 20x :
:     20      :
The light is at
The person is: .
His distance from the wall is: .
This distance is decreasing at: . m/sec.
The height of his shadow is: .
From the similar right triangles, we have:
. .
Differentiate with respect to time: .
When
The height of the shadow is decreasing at meters per second.