Trigonometry
posted by may .
Two sides of a triangle are 5 and 8 units respectively. If the included angle is changing at rate of one radian, at what rate is the third side changing when the included angle is 60 degrees?

One radian is not a rate. It is an angle. Look at the problem statement again.

drwls is right, you must have left out the units of time.
(e.g. ... is changing at a rate of one radian per minute )
In that case, use the cosine law equation as your starting point
let the third side be x and the angle be Ø
x^2 = 5^2 + 8^2  2(5)(8)cosØ
2x dx/dt = 0 + 0 + 80 sinØ dØ/dt
from the original equation, find x , then sub in the given values
pretty easy after that.