WEBVTT

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Two points are enough to pin down a straight line.

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Let us find the equation of the line through one comma two and three comma eight.

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Slope is the change in y divided by the change in x.

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Eight minus two is six, three minus one is two, so the slope is three.

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In slope intercept form the line is y equals three x plus b.

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Substitute the point one comma two: two equals three plus b.

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So b is negative one.

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The line is y equals three x minus one.

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On the graph, the line rises three units for every one unit to the right.

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It passes through both points, and crosses the y axis at negative one.

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To recap: find the slope, substitute one point to get the intercept, and check with the other point. Three times three minus one is eight, as it should be.