Question

At 1 P.M., ship A is 150 km west of ship B. Ship A is sailing east at 35 km/h and ship B is sailing north at 25 km/h. How fast is the distance between the ships changing at 5 P.M.?

Answer 1

The distance between the ships is changing at a rate of 21.393 km/h

Step 1: First, you must draw an image to help better understand the problem 

The reason that ship A is moving -35km/h is because ship B acts as an "origin" and as things move closer to the "origin" they become negative and as things move away from the "origin" they become positive.

We know that the rate of Ship A is expressed as dAdt=−35 and the rate of Ship B is expressed as dBdt=25

Step 2: We want to figure out how many km Ship A and Ship B traveled in 4 hours

ShipA:−35kmh⋅4hr=−140km

Now if we take this −140km and add it to the 150km we can see that Ship A is now only 10km away from where Ship B began

ShipB:25kmh⋅4hr=100km

This means that Ship B is now 100km from where it originally started

We can now redraw our information

Step 3: We must use the Pythagorean Theorem to find the third side of the triangle

x2+y2=z2→102+1002=z2

z=√102+1002

Step 4: Now that we know z, we must differentiate the original equation in order to find the rate at which z in changing

x2+y2=z2→2xdxdt+2ydydt=2zdzdt

We can simplify by canceling out the 2's

xdxdt+ydydt=zdzdt

Step 5: Write down all the information and then plug into equation

x=10anddxdt=−35
y=100anddydt=25
z=√102+1002anddzdt=?

Now plug in the information

xdxdt+ydydt=zdzdt

10(−35)+100(25)=√102+1002dzdt

−350+2500=√102+1002dzdt

2150=√102+1002dzdt

21.393=dzdt

The distance between the ships are increasing at a rate of 21.393 km/h