document.write( "Question 1069666: Det. Robert Goren is investigating a homicide, and the medical examiner tells him that when he arrived at 6:00 p.m., the body had a temperature of 95.3 degrees Fahrenheit. By the time Nichols had arrived an hour later (at 7:00), he checks again and determines the temperature to be 94.9 degrees. The ambient air temperature is 90 degrees. Use the equation 𝑇(𝑡) = 𝑎𝑒^(−𝑘𝑡) + 90 to determine when the victim was killed. (The normal body temperature is 98.6 degrees. Round k to 3 decimal places and find the time of death to the nearest minute.\r
\n" ); document.write( "\n" ); document.write( "I'm really confused on how to go about this problem, any help would be greatly appreciated!
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Algebra.Com's Answer #684849 by KMST(5328)\"\" \"About 
You can put this solution on YOUR website!
\"T%28t%29=a%2Ae%5E%28-kt%29%2B90\" makes some sense.
\n" ); document.write( "\"T%28t%29\" would be the body temperature in degrees at some time \"t\".
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\n" ); document.write( "ONE WAY TO SOLVE:
\n" ); document.write( "We define \"t\"= time in minutes after 6:00 p.m.;
\n" ); document.write( "substitute \"T%280%29=95.3\" and \"T%2860%29=94.9\" to find \"a\" and \"k\",
\n" ); document.write( "and then use \"T%28t%29=98.6\" to find the time the victim died.
\n" ); document.write( "Adding the negative time we find to 6:00 p.m. gives us the murder time.
\n" ); document.write( "It gets easier if we solve \"T%28t%29=a%2Ae%5E%28-kt%29%2B90\" for what we want to start,
\n" ); document.write( "and then just plug stuff in.
\n" ); document.write( "\"a=%28T-90%29%2Fe%5E%28-kt%29\"
\n" ); document.write( "\"k=-%281%2Ft%29ln%28%28T-90%29%2Fa%29\"
\n" ); document.write( "\"t=-%281%2Fk%29ln%28%28T-90%29%2Fa%29\"
\n" ); document.write( "At 6:00 p.m. \"t=0\"<-->\"-kt=0\" and \"T=95.3\" , so
\n" ); document.write( "\"a=%2895.3-90%29%2Fe%5E0\"-->\"a=5.3%2F1\"-->\"highlight%28a=5.3%29\"
\n" ); document.write( "At 7:00 p.m. \"t=60\" and \"T=94.9\" , so
\n" ); document.write( "\"k=-%281%2F60%29ln%28%2894.9-90%29%2F5.3%29\"-->\"k=-%281%2F60k%29ln%284.9%2F5.3%29\"-->\"k=-%281%2F60%29%28-0.7850%29\"-->\"highlight%28k=0.001308%29\"
\n" ); document.write( "At the time the victim died, \"T=98.6\" , so
\n" ); document.write( "\"t=-%281%2F60%29ln%28%2898.6-90%29%2F5.3%29\"-->\"t=-%281%2F60%29ln%288.6%2F5.3%29\"-->\"t=-%281%2F60%290.4840\"-->\"highlight%28t=-370%29\" (rounded to the nearest minute)
\n" ); document.write( "Since \"370minutes=6hours%2B11minutes\" ,
\n" ); document.write( "time of death = \"%226%3A00+PM%22-%226%3A11%22=highlight%28%2211%3A49+AM%22%29\" .
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\n" ); document.write( "EXPLANATION AND ALTERNATE WAY TO SOLVE:
\n" ); document.write( "You may have heard in class that
\n" ); document.write( "\"y=e%5E%28-kx%29\" for some \"k%3E0\" , \"graph%28200%2C200%2C-0.1%2C0.9%2C-0.1%2C0.9%2C0.8%2Ae%5E%28-%28sqrt%285x%29%5E2%29%29%29\"
\n" ); document.write( "models exponential decay;
\n" ); document.write( "that it asymptotically approaches \"y=0\",
\n" ); document.write( "or even that \"lim%28x-%3Einfinity%2Ce%5E%28-kx%29%29=0\" .
\n" ); document.write( "
\n" ); document.write( "\"T%28t%29=a%2Ae%5E%28-kt%29%2B90\" (with some \"a%3E0\" and \"k%3E0\" too) makes sense.
\n" ); document.write( "It would have \"T%280%29=a%2Ae%5E0%2B90=a%2A1%2B90=a%2B90\" ,
\n" ); document.write( "and would decrease towards \"90\" , which is the ambient temperature in degrees.
\n" ); document.write( "Nothing else is specified about that function,
\n" ); document.write( "so we will say that
\n" ); document.write( "\"t\"= time since 6:00 p.m., in minutes.
\n" ); document.write( "So, \"T%280%29=95.3\" tells us that
\n" ); document.write( "\"a%2B90=95.3\" --> \"a=95.3-90\" ---> \"highlight%28a=5.3%29\"
\n" ); document.write( "An hour later, \"t=60\" ,
\n" ); document.write( "and \"T%2860%29=94.9\" tells us that
\n" ); document.write( "\"a%2Ae%5E%28-60k%29%2B90=94.9\" --> \"5.3%2Ae%5E%28-60k%29=94.9-90\" --> \"5.3%2Ae%5E%28-60k%29=4.9\"
\n" ); document.write( "--> \"e%5E%28-60k%29=4.9%2F5.3\" --> \"-60k=ln%280.9245%29\" --> \"-60k=-0.07850\"
\n" ); document.write( "--> \"k=%28-0.07850%29%2F%28-60%29\" --> \"highlight%28k=0.001308%29\"
\n" ); document.write( "Now we have the function
\n" ); document.write( "\"T%28t%29=5.3e%5E%28-0.001308t%29%2B90\"
\n" ); document.write( "and we want to find \"t\" for \"T%28t%29=98.6\"
\n" ); document.write( "\"98.6=5.3e%5E%28-0.001308t%29%2B90\"
\n" ); document.write( "\"98.6-90=5.3e%5E%28-0.001308t%29\"
\n" ); document.write( "\"8.6=5.3e%5E%28-0.001308t%29\"
\n" ); document.write( "\"8.6%2F5.3=e%5E%28-0.001308t%29\"
\n" ); document.write( "\"ln%288.6%2F5.3%29=-0.001308t\"
\n" ); document.write( "\"t=ln%288.6%2F5.3%29%2F%28-0.001308%29\"
\n" ); document.write( "\"highlight%28t=-370%29\" (rounded to the nearest minute)
\n" ); document.write( "Since \"370minutes=6hours%2B11minutes\" ,
\n" ); document.write( "time of death = \"%226%3A00+PM%22-%226%3A11%22=highlight%28%2211%3A49+AM%22%29\" .
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