standard_description: Use similar triangles to explain why the slope m is the same between any two distinct points on a non-vertical line in the coordinate plane; derive the equation y = mx for a line through the origin and the equation y = mx + b for a line intercepting the vertical axis at b.
problem_activity: Problem 1
A scientist plants a seed in a laboratory and tracks the growth of the plant. The height of the plant in centimeters is proportional to the number of days since it was planted.
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a.   Explain what point B represents in context of the situation.
b.   At what rate is the plant growing? Indicate this as a point on the graph.
c.   Write an equation that represents the relationship between days and height.
d.   If the plant continues to grow at this rate, how tall will it be after 12 days?
e.   If the plant continues to grow at this rate, when will it be 6 cm tall?
Problem 2
Match each table to its equation.
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i.
$${y = {1 \over 2}x}$$
ii.
$${y = {2 \over 3}x}$$
iii.
$${y = 2x}$$
iv.
$${y=3x}$$
Problem 3
The cost you pay for limes is proportional to the number of limes you buy. Four different stores sell limes for different amounts, as shown in the graphs, table, and equation below. Which store should you go to if you want to pay the least amount for limes? Explain your answer.
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