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
