Simple word problem: Chain rule

[michonamona]

Homework Statement

One side of a triangle is increasing at a rate of 3cm/s and a second side is decreasing at a rate of 2cm/s. If the area of the triangle remain constant, at what rate does the angle between the sides change when the first side is 20cm long and the second side is 30cm, and the angle is pi/6?

Homework Equations

The Attempt at a Solution

My only question is, what is the formula for the angle between the two side? I know that it will be a function of the length of the two side, but I don't know how to find the explicit expression.

Thanks,
M

 

[Char. Limit]

Sines and cosines will serve you well.

 

[michonamona]

Thanks, would you be so kind as to elaborate on your answer? I would appreciate it. My background in trigonometry is weak; although I do know how to use them on a right triangle, but in this case, I'm not quite certain.

 

[Dick]

Drop a perpendicular from one side to the other. That gives you a right triangle doesn't it? In that case you said you can do it. Then use area=(1/2)*base*height. If the two sides are a and b and the included angle is theta, what does that give you?