Question

A candle is 20 in. tall after burning for 3 hours. After 5 hours, it is 19 in. tall. Write a linear equation to model the relationship between height h of the candle and time t. Predict how tall the candle will be after burning 8 hours.

Answer 1

$\begin{array}{ccll} \stackrel{hours}{t}&\stackrel{height}{h}\\ \cline{1-2} 3&20\\ 5&19 \end{array}$  with those values provided from the table, let's get its equation

$(\stackrel{x_1}{3}~,~\stackrel{y_1}{20})\qquad (\stackrel{x_2}{5}~,~\stackrel{y_2}{19}) ~\hfill \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{19}-\stackrel{y1}{20}}}{\underset{run} {\underset{x_2}{5}-\underset{x_1}{3}}} \implies \cfrac{19 -20}{5 -3} \implies -\cfrac{ 1 }{ 2 }$

$\begin{array}{|c|ll} \cline{1-1} \textit{point-slope form}\\ \cline{1-1} \\ y-y_1=m(x-x_1) \\\\ \cline{1-1} \end{array}\implies y-\stackrel{y_1}{20}=\stackrel{m}{-\cfrac{ 1 }{ 2 }}(x-\stackrel{x_1}{3}) \implies y-20=-\cfrac{ 1 }{ 2 }x+\cfrac{3}{2} \\\\\\ y=-\cfrac{ 1 }{ 2 }x+\cfrac{3}{2}+20\implies y=-\cfrac{ 1 }{ 2 }x+\cfrac{43}{2}\implies \LARGE \begin{array}{llll} h=-\cfrac{ 1 }{ 2 }t+\cfrac{43}{2} \end{array}$

how talll will the candle be after 8 hours?  namely when t = 8.

$h=-\cfrac{ 1 }{ 2 }(8)+\cfrac{43}{2}\implies h=-4+\cfrac{43}{2}\implies h=\cfrac{35}{2}\implies \LARGE \begin{array}{llll} h=17\frac{1}{2} \end{array}$