Question 317526
Let's call A, the distance from the intersection to car A.
Let's call B, the distance from the intersection to car B.
The distance between the cars is D.
A and B form a right triangle with D as the hypotenuse.
{{{D^2=A^2+B^2}}}
If you differentiate with respect to time,
{{{2D*(dD/dt)=2A*(dA/dt)+2B*(dB/dt)}}}
After 2 hours,
{{{A=-80+80*2=80}}}
{{{B=-260+100*2=-60}}}
{{{dA/dt=80}}}
{{{dB/dt=100}}}
{{{D^2=A^2+B^2=80^2+60^2=100^2}}}
{{{D=100}}}
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.
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{{{2D*dD/dt=2A*dA/dt+2*B*dB/dt}}}
{{{2(100)dD/dt=2*80*80+2(-60)(80)}}}
{{{200*dD/dt=2(80)(80-60)=3200}}}
{{{dD/dt=3200/200}}}
{{{dD/dt=16}}}
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.
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The only issue remaining is the sign of the rate of change.
Distance A is growing while Distance B is diminishing (A is growing slower than B). 
Distance D is therefore diminishing. 
{{{highlight(dD/dt=-16)}}} km/h