### Ang Yew Keong's answer to Christine Goh's Secondary 4 A Maths question.

The working on finding the rate of change of x is done correctly.

However, the perimeter being a function of x is not considered.

Perimeter is 10x,

dP/dt = (dP/dx)*(dx/dt)

= (10)*(1cm/s) = 10cm/s

(the units for dA/dx found in this answer should be cm instead)