Two sides of the triangle are  m and  m.

Angle between them is increasing at a rate of / min.

In ,

m,  m and consider .

Consider the angle between the fixed sides as .

The increasing rate of angle .

Convert the above value into radians.

Apply cosine rule to the .

Differentiate on each side with respect to .

                          (Since and are constant values)

.

Find value when angle between the two fixed sides is .

Substitute values of  and in .

Therefore, third side is increasing at the rate of m/min.

The length of  third side is increasing at the rate of m/min.