###### Example4.8.19

Two trees were planted in the same year, and their growth over time is modeled by the two lines in Figure 4.8.20. Use linear equations to model each tree's growth, and interpret their meanings in this context.

We can see Tree 1's equation is $$y=\frac{2}{3}x+2\text{,}$$ and Tree 2's equation is $$y=\frac{2}{3}x+5\text{.}$$ Tree 1 was $$2$$ feet tall when it was planted, and Tree 2 was $$5$$ feet tall when it was planted. Both trees have been growing at the same rate, $$\frac{2}{3}$$ feet per year, or $$2$$ feet every $$3$$ years.

An important observation right now is that those two lines are parallel. Why? For lines with positive slopes, the bigger a line's slope, the steeper the line is slanted. As a result, if two lines have the same slope, they are slanted at the same angle, thus they are parallel.

in-context