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Answer:
[tex]22.21\ \text{mph}[/tex]
[tex]22.2\ \text{mph}[/tex]
Step-by-step explanation:
[tex]\dfrac{da}{dt}=13\ \text{mph}[/tex]
[tex]\dfrac{db}{dt}=18\ \text{mph}[/tex]
[tex]\dfrac{dc}{dt}[/tex] = Rate at which the distance between the two people is changing
At 25 minutes
[tex]a=13\times \dfrac{25}{60}=5.417\ \text{miles}[/tex]
[tex]b=18\times \dfrac{25}{60}=7.5\ \text{miles}[/tex]
[tex]c=\sqrt{5.417^2+7.5^2}\\\Rightarrow c=9.25\ \text{miles}[/tex]
Distance between the two people is
[tex]c^2=a^2+b^2[/tex]
Differentiating with respect to time we get
[tex]c\dfrac{dc}{dt}=a\dfrac{da}{dt}+b\dfrac{db}{dt}\\\Rightarrow \dfrac{dc}{dt}=\dfrac{a\dfrac{da}{dt}+b\dfrac{db}{dt}}{c}\\\Rightarrow \dfrac{dc}{dt}=\dfrac{5.417\times 13+7.5\times 18}{9.25}\\\Rightarrow \dfrac{dc}{dt}=22.21\ \text{mph}[/tex]
Distance between the two people is changing at a rate of [tex]22.21\ \text{mph}[/tex].
At 35 minutes
[tex]a=13\times \dfrac{35}{60}=7.583\ \text{miles}[/tex]
[tex]b=18\times \dfrac{35}{60}=10.5\ \text{miles}[/tex]
[tex]c=\sqrt{7.583^2+10.5^2}\\\Rightarrow c=12.95\ \text{miles}[/tex]
Distance between the two people is
[tex]c^2=a^2+b^2[/tex]
Differentiating with respect to time we get
[tex]c\dfrac{dc}{dt}=a\dfrac{da}{dt}+b\dfrac{db}{dt}\\\Rightarrow \dfrac{dc}{dt}=\dfrac{a\dfrac{da}{dt}+b\dfrac{db}{dt}}{c}\\\Rightarrow \dfrac{dc}{dt}=\dfrac{7.583\times 13+10.5\times 18}{12.95}\\\Rightarrow \dfrac{dc}{dt}=22.2\ \text{mph}[/tex]
Distance between the two people is changing at a rate of [tex]22.2\ \text{mph}[/tex].
