Calculus-Derivative+word problem. help:( 1. The problem statement, all variables and given/known data Two snails are sliming along my square patio's edges. One is on the north edge, 120cm away from the northeast corner and travelling directly east at the snail's pace of 6cm/min. The other is on the east edge, 50cm away from the northeast corner and travelling north at 4cm/min. At what rate is the distance between the snails changing? 2. Relevant equations I do not know what equation to use. But I used root(a2+b2)=c But I don't think I am using the right equation. 3. The attempt at a solution let time be t, and upside and right be positive. snail no.1:f(x, y)=xi+yi, x=6t-120, y=0 snail no.2:g(x, y)=xi+yi, x=0, y=4t-50 d(distance)=root[(6t-120)2+(-4t+50)2] = (52t2-1120t+16900)1/2 So this was the answer I got. But I am now learning derivative and increasing and decreasing function. So I don't think this should be solved this way. There must be something related to derivative when solving this problem. My prof must have gave us this problem in order to let us solve with the method which we are learning, isn't it? And I also think the answer is wrong. What should I do? How do I solve this prob? And if I solved in right way, should I take the derivative of my answer to be the real answer of my problem?