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A coffee with temperature \(\displaystyle 167^\circ\) is left in a room with temperature \(\displaystyle 70^\circ\). After 10 minutes the temperature of the coffee is \(\displaystyle 161^\circ\), how long until the coffee is \(\displaystyle 156^\circ\)?
Using \(\displaystyle T = T_0+(T_1-T_0)e^{kt}\) gives \(\displaystyle T = 70+(167-70)e^{kt}= 70+97e^{kt}\). Using the point \(\displaystyle (10, 161)\) gives \(\displaystyle 161= 70+97e^{k(10)}\). Isolating the exponential gives \(\displaystyle \frac{91}{97}=e^{10k}\). Solving for \(\displaystyle k\) gives \(\displaystyle k=\frac{\ln{\left(\frac{91}{97} \right)}}{10}\). Substuting \(\displaystyle k\) back into the equation gives \(\displaystyle T = 70+97e^{\frac{\ln{\left(\frac{91}{97} \right)}}{10}t}\) and simplifying gives \(\displaystyle T = 97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70\). Using \(\displaystyle T\) gives the equation \(\displaystyle 156=97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70\). Isolating the exponential gives \(\displaystyle \frac{86}{97}=\left(\frac{91}{97}\right)^{\frac{t}{10}}\). Taking the natural logarithm of both sides and solving for \(\displaystyle t\) gives \(\displaystyle t = \frac{10 \ln{\left(\frac{86}{97} \right)}}{\ln{\left(\frac{91}{97} \right)}}\approx 19\) minutes.
\begin{question}A coffee with temperature $167^\circ$ is left in a room with temperature $70^\circ$. After 10 minutes the temperature of the coffee is $161^\circ$, how long until the coffee is $156^\circ$? \soln{9cm}{Using $T = T_0+(T_1-T_0)e^{kt}$ gives $T = 70+(167-70)e^{kt}= 70+97e^{kt}$. Using the point $(10, 161)$ gives $161= 70+97e^{k(10)}$. Isolating the exponential gives $\frac{91}{97}=e^{10k}$. Solving for $k$ gives $k=\frac{\ln{\left(\frac{91}{97} \right)}}{10}$. Substuting $k$ back into the equation gives $T = 70+97e^{\frac{\ln{\left(\frac{91}{97} \right)}}{10}t}$ and simplifying gives $T = 97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70$. Using $T$ gives the equation $156=97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70$. Isolating the exponential gives $\frac{86}{97}=\left(\frac{91}{97}\right)^{\frac{t}{10}}$. Taking the natural logarithm of both sides and solving for $t$ gives $t = \frac{10 \ln{\left(\frac{86}{97} \right)}}{\ln{\left(\frac{91}{97} \right)}}\approx 19$ minutes. } \end{question}
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<p> <p>A coffee with temperature <img class="equation_image" title=" \displaystyle 167^\circ " src="/equation_images/%20%5Cdisplaystyle%20167%5E%5Ccirc%20" alt="LaTeX: \displaystyle 167^\circ " data-equation-content=" \displaystyle 167^\circ " /> is left in a room with temperature <img class="equation_image" title=" \displaystyle 70^\circ " src="/equation_images/%20%5Cdisplaystyle%2070%5E%5Ccirc%20" alt="LaTeX: \displaystyle 70^\circ " data-equation-content=" \displaystyle 70^\circ " /> . After 10 minutes the temperature of the coffee is <img class="equation_image" title=" \displaystyle 161^\circ " src="/equation_images/%20%5Cdisplaystyle%20161%5E%5Ccirc%20" alt="LaTeX: \displaystyle 161^\circ " data-equation-content=" \displaystyle 161^\circ " /> , how long until the coffee is <img class="equation_image" title=" \displaystyle 156^\circ " src="/equation_images/%20%5Cdisplaystyle%20156%5E%5Ccirc%20" alt="LaTeX: \displaystyle 156^\circ " data-equation-content=" \displaystyle 156^\circ " /> ?</p> </p>
<p> <p>Using <img class="equation_image" title=" \displaystyle T = T_0+(T_1-T_0)e^{kt} " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%20T_0%2B%28T_1-T_0%29e%5E%7Bkt%7D%20" alt="LaTeX: \displaystyle T = T_0+(T_1-T_0)e^{kt} " data-equation-content=" \displaystyle T = T_0+(T_1-T_0)e^{kt} " /> gives <img class="equation_image" title=" \displaystyle T = 70+(167-70)e^{kt}= 70+97e^{kt} " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2070%2B%28167-70%29e%5E%7Bkt%7D%3D%2070%2B97e%5E%7Bkt%7D%20" alt="LaTeX: \displaystyle T = 70+(167-70)e^{kt}= 70+97e^{kt} " data-equation-content=" \displaystyle T = 70+(167-70)e^{kt}= 70+97e^{kt} " /> . Using the point <img class="equation_image" title=" \displaystyle (10, 161) " src="/equation_images/%20%5Cdisplaystyle%20%2810%2C%20161%29%20" alt="LaTeX: \displaystyle (10, 161) " data-equation-content=" \displaystyle (10, 161) " /> gives <img class="equation_image" title=" \displaystyle 161= 70+97e^{k(10)} " src="/equation_images/%20%5Cdisplaystyle%20161%3D%2070%2B97e%5E%7Bk%2810%29%7D%20" alt="LaTeX: \displaystyle 161= 70+97e^{k(10)} " data-equation-content=" \displaystyle 161= 70+97e^{k(10)} " /> . Isolating the exponential gives <img class="equation_image" title=" \displaystyle \frac{91}{97}=e^{10k} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7B91%7D%7B97%7D%3De%5E%7B10k%7D%20" alt="LaTeX: \displaystyle \frac{91}{97}=e^{10k} " data-equation-content=" \displaystyle \frac{91}{97}=e^{10k} " /> . Solving for <img class="equation_image" title=" \displaystyle k " src="/equation_images/%20%5Cdisplaystyle%20k%20" alt="LaTeX: \displaystyle k " data-equation-content=" \displaystyle k " /> gives <img class="equation_image" title=" \displaystyle k=\frac{\ln{\left(\frac{91}{97} \right)}}{10} " src="/equation_images/%20%5Cdisplaystyle%20k%3D%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%20%5Cright%29%7D%7D%7B10%7D%20" alt="LaTeX: \displaystyle k=\frac{\ln{\left(\frac{91}{97} \right)}}{10} " data-equation-content=" \displaystyle k=\frac{\ln{\left(\frac{91}{97} \right)}}{10} " /> . Substuting <img class="equation_image" title=" \displaystyle k " src="/equation_images/%20%5Cdisplaystyle%20k%20" alt="LaTeX: \displaystyle k " data-equation-content=" \displaystyle k " /> back into the equation gives <img class="equation_image" title=" \displaystyle T = 70+97e^{\frac{\ln{\left(\frac{91}{97} \right)}}{10}t} " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2070%2B97e%5E%7B%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%20%5Cright%29%7D%7D%7B10%7Dt%7D%20" alt="LaTeX: \displaystyle T = 70+97e^{\frac{\ln{\left(\frac{91}{97} \right)}}{10}t} " data-equation-content=" \displaystyle T = 70+97e^{\frac{\ln{\left(\frac{91}{97} \right)}}{10}t} " /> and simplifying gives <img class="equation_image" title=" \displaystyle T = 97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2097%20%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B10%7D%7D%20%2B%2070%20" alt="LaTeX: \displaystyle T = 97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " data-equation-content=" \displaystyle T = 97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " /> . Using <img class="equation_image" title=" \displaystyle T " src="/equation_images/%20%5Cdisplaystyle%20T%20" alt="LaTeX: \displaystyle T " data-equation-content=" \displaystyle T " /> gives the equation <img class="equation_image" title=" \displaystyle 156=97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " src="/equation_images/%20%5Cdisplaystyle%20156%3D97%20%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B10%7D%7D%20%2B%2070%20" alt="LaTeX: \displaystyle 156=97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " data-equation-content=" \displaystyle 156=97 \left(\frac{91}{97}\right)^{\frac{t}{10}} + 70 " /> . Isolating the exponential gives <img class="equation_image" title=" \displaystyle \frac{86}{97}=\left(\frac{91}{97}\right)^{\frac{t}{10}} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7B86%7D%7B97%7D%3D%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B10%7D%7D%20" alt="LaTeX: \displaystyle \frac{86}{97}=\left(\frac{91}{97}\right)^{\frac{t}{10}} " data-equation-content=" \displaystyle \frac{86}{97}=\left(\frac{91}{97}\right)^{\frac{t}{10}} " /> . Taking the natural logarithm of both sides and solving for <img class="equation_image" title=" \displaystyle t " src="/equation_images/%20%5Cdisplaystyle%20t%20" alt="LaTeX: \displaystyle t " data-equation-content=" \displaystyle t " /> gives <img class="equation_image" title=" \displaystyle t = \frac{10 \ln{\left(\frac{86}{97} \right)}}{\ln{\left(\frac{91}{97} \right)}}\approx 19 " src="/equation_images/%20%5Cdisplaystyle%20t%20%3D%20%5Cfrac%7B10%20%5Cln%7B%5Cleft%28%5Cfrac%7B86%7D%7B97%7D%20%5Cright%29%7D%7D%7B%5Cln%7B%5Cleft%28%5Cfrac%7B91%7D%7B97%7D%20%5Cright%29%7D%7D%5Capprox%2019%20" alt="LaTeX: \displaystyle t = \frac{10 \ln{\left(\frac{86}{97} \right)}}{\ln{\left(\frac{91}{97} \right)}}\approx 19 " data-equation-content=" \displaystyle t = \frac{10 \ln{\left(\frac{86}{97} \right)}}{\ln{\left(\frac{91}{97} \right)}}\approx 19 " /> minutes. </p> </p>