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A coffee with temperature \(\displaystyle 154^\circ\) is left in a room with temperature \(\displaystyle 66^\circ\). After 15 minutes the temperature of the coffee is \(\displaystyle 147^\circ\), how long until the coffee is \(\displaystyle 139^\circ\)?


Using \(\displaystyle T = T_0+(T_1-T_0)e^{kt}\) gives \(\displaystyle T = 66+(154-66)e^{kt}= 66+88e^{kt}\). Using the point \(\displaystyle (15, 147)\) gives \(\displaystyle 147= 66+88e^{k(15)}\). Isolating the exponential gives \(\displaystyle \frac{81}{88}=e^{15k}\). Solving for \(\displaystyle k\) gives \(\displaystyle k=\frac{\ln{\left(\frac{81}{88} \right)}}{15}\). Substuting \(\displaystyle k\) back into the equation gives \(\displaystyle T = 66+88e^{\frac{\ln{\left(\frac{81}{88} \right)}}{15}t}\) and simplifying gives \(\displaystyle T = 88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66\). Using \(\displaystyle T\) gives the equation \(\displaystyle 139=88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66\). Isolating the exponential gives \(\displaystyle \frac{73}{88}=\left(\frac{81}{88}\right)^{\frac{t}{15}}\). Taking the natural logarithm of both sides and solving for \(\displaystyle t\) gives \(\displaystyle t = \frac{15 \ln{\left(\frac{73}{88} \right)}}{\ln{\left(\frac{81}{88} \right)}}\approx 34\) minutes.

Download \(\LaTeX\)

\begin{question}A coffee with temperature $154^\circ$ is left in a room with temperature $66^\circ$. After 15 minutes the temperature of the coffee is $147^\circ$, how long until the coffee is $139^\circ$?
    \soln{9cm}{Using $T = T_0+(T_1-T_0)e^{kt}$ gives $T = 66+(154-66)e^{kt}= 66+88e^{kt}$. Using the point $(15, 147)$ gives $147= 66+88e^{k(15)}$. Isolating the exponential gives $\frac{81}{88}=e^{15k}$. Solving for $k$ gives $k=\frac{\ln{\left(\frac{81}{88} \right)}}{15}$.  Substuting $k$ back into the equation gives $T = 66+88e^{\frac{\ln{\left(\frac{81}{88} \right)}}{15}t}$ and simplifying gives $T = 88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66$. Using $T$ gives the equation $139=88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66$.  Isolating the exponential gives $\frac{73}{88}=\left(\frac{81}{88}\right)^{\frac{t}{15}}$. Taking the natural logarithm of both sides and solving for $t$ gives $t = \frac{15 \ln{\left(\frac{73}{88} \right)}}{\ln{\left(\frac{81}{88} \right)}}\approx 34$ minutes. }

\end{question}

Download Question and Solution Environment\(\LaTeX\)
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HTML for Canvas
<p> <p>A coffee with temperature  <img class="equation_image" title=" \displaystyle 154^\circ " src="/equation_images/%20%5Cdisplaystyle%20154%5E%5Ccirc%20" alt="LaTeX:  \displaystyle 154^\circ " data-equation-content=" \displaystyle 154^\circ " />  is left in a room with temperature  <img class="equation_image" title=" \displaystyle 66^\circ " src="/equation_images/%20%5Cdisplaystyle%2066%5E%5Ccirc%20" alt="LaTeX:  \displaystyle 66^\circ " data-equation-content=" \displaystyle 66^\circ " /> . After 15 minutes the temperature of the coffee is  <img class="equation_image" title=" \displaystyle 147^\circ " src="/equation_images/%20%5Cdisplaystyle%20147%5E%5Ccirc%20" alt="LaTeX:  \displaystyle 147^\circ " data-equation-content=" \displaystyle 147^\circ " /> , how long until the coffee is  <img class="equation_image" title=" \displaystyle 139^\circ " src="/equation_images/%20%5Cdisplaystyle%20139%5E%5Ccirc%20" alt="LaTeX:  \displaystyle 139^\circ " data-equation-content=" \displaystyle 139^\circ " /> ?</p> </p>
HTML for Canvas
<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 = 66+(154-66)e^{kt}= 66+88e^{kt} " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2066%2B%28154-66%29e%5E%7Bkt%7D%3D%2066%2B88e%5E%7Bkt%7D%20" alt="LaTeX:  \displaystyle T = 66+(154-66)e^{kt}= 66+88e^{kt} " data-equation-content=" \displaystyle T = 66+(154-66)e^{kt}= 66+88e^{kt} " /> . Using the point  <img class="equation_image" title=" \displaystyle (15, 147) " src="/equation_images/%20%5Cdisplaystyle%20%2815%2C%20147%29%20" alt="LaTeX:  \displaystyle (15, 147) " data-equation-content=" \displaystyle (15, 147) " />  gives  <img class="equation_image" title=" \displaystyle 147= 66+88e^{k(15)} " src="/equation_images/%20%5Cdisplaystyle%20147%3D%2066%2B88e%5E%7Bk%2815%29%7D%20" alt="LaTeX:  \displaystyle 147= 66+88e^{k(15)} " data-equation-content=" \displaystyle 147= 66+88e^{k(15)} " /> . Isolating the exponential gives  <img class="equation_image" title=" \displaystyle \frac{81}{88}=e^{15k} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7B81%7D%7B88%7D%3De%5E%7B15k%7D%20" alt="LaTeX:  \displaystyle \frac{81}{88}=e^{15k} " data-equation-content=" \displaystyle \frac{81}{88}=e^{15k} " /> . 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{81}{88} \right)}}{15} " src="/equation_images/%20%5Cdisplaystyle%20k%3D%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%20%5Cright%29%7D%7D%7B15%7D%20" alt="LaTeX:  \displaystyle k=\frac{\ln{\left(\frac{81}{88} \right)}}{15} " data-equation-content=" \displaystyle k=\frac{\ln{\left(\frac{81}{88} \right)}}{15} " /> .  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 = 66+88e^{\frac{\ln{\left(\frac{81}{88} \right)}}{15}t} " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2066%2B88e%5E%7B%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%20%5Cright%29%7D%7D%7B15%7Dt%7D%20" alt="LaTeX:  \displaystyle T = 66+88e^{\frac{\ln{\left(\frac{81}{88} \right)}}{15}t} " data-equation-content=" \displaystyle T = 66+88e^{\frac{\ln{\left(\frac{81}{88} \right)}}{15}t} " />  and simplifying gives  <img class="equation_image" title=" \displaystyle T = 88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D%2088%20%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B15%7D%7D%20%2B%2066%20" alt="LaTeX:  \displaystyle T = 88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " data-equation-content=" \displaystyle T = 88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " /> . 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 139=88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " src="/equation_images/%20%5Cdisplaystyle%20139%3D88%20%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B15%7D%7D%20%2B%2066%20" alt="LaTeX:  \displaystyle 139=88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " data-equation-content=" \displaystyle 139=88 \left(\frac{81}{88}\right)^{\frac{t}{15}} + 66 " /> .  Isolating the exponential gives  <img class="equation_image" title=" \displaystyle \frac{73}{88}=\left(\frac{81}{88}\right)^{\frac{t}{15}} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7B73%7D%7B88%7D%3D%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B15%7D%7D%20" alt="LaTeX:  \displaystyle \frac{73}{88}=\left(\frac{81}{88}\right)^{\frac{t}{15}} " data-equation-content=" \displaystyle \frac{73}{88}=\left(\frac{81}{88}\right)^{\frac{t}{15}} " /> . 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{15 \ln{\left(\frac{73}{88} \right)}}{\ln{\left(\frac{81}{88} \right)}}\approx 34 " src="/equation_images/%20%5Cdisplaystyle%20t%20%3D%20%5Cfrac%7B15%20%5Cln%7B%5Cleft%28%5Cfrac%7B73%7D%7B88%7D%20%5Cright%29%7D%7D%7B%5Cln%7B%5Cleft%28%5Cfrac%7B81%7D%7B88%7D%20%5Cright%29%7D%7D%5Capprox%2034%20" alt="LaTeX:  \displaystyle t = \frac{15 \ln{\left(\frac{73}{88} \right)}}{\ln{\left(\frac{81}{88} \right)}}\approx 34 " data-equation-content=" \displaystyle t = \frac{15 \ln{\left(\frac{73}{88} \right)}}{\ln{\left(\frac{81}{88} \right)}}\approx 34 " />  minutes. </p> </p>