Chapter 13.5, Problem 35ES

Let a and b denote two sides of a triangle and let $\theta$ denote the included angle. Suppose that $a,b,\text{and}\theta$ vary with time in such a way that the area of the triangle remains constant. At a certain instant $a=5\text{cm},b=4\text{cm},\text{and}\theta =\pi /6$ radians, and at that instant both a and b are increasing at a rate of 3 cm/s. Estimate the rate at which $\theta$ is changing at that instant.