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Deriving the Point-Slope Form from the Slope Formula

The point-slope form of a line's equation can be derived directly from the slope formula. Starting with a line of slope mm that passes through a known point (x1,y1)(x_1, y_1) and any other point (x,y)(x, y), the slope formula gives: m=yy1xx1m = \frac{y - y_1}{x - x_1} To eliminate the fraction, multiply both sides by (xx1)(x - x_1): m(xx1)=yy1xx1(xx1)m(x - x_1) = \frac{y - y_1}{x - x_1} \cdot (x - x_1) The right side simplifies because the (xx1)(x - x_1) factors cancel: m(xx1)=yy1m(x - x_1) = y - y_1 Finally, rewriting with the yy terms on the left yields the point-slope form: yy1=m(xx1)y - y_1 = m(x - x_1) This derivation shows that the point-slope form is simply a rearrangement of the slope formula — the same relationship between slope, rise, and run, rewritten so that it can be used as an equation of the line rather than just a calculation of slope.

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Updated 2026-05-03

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