**Unformatted text preview: **and continues with slope 0.33. See ﬁgure 1.2 . 10000 20000 30000 50000 100000 ..................................................................................................................................................................................................................... • • Figure 1.2 Tax vs. income. The most familiar form of the equation of a straight line is: y = mx + b . Here m is the slope of the line: if you increase x by 1, the equation tells you that you have to increase y by m . If you increase x by Δ x , then y increases by Δ y = m Δ x . The number b is called the y-intercept , because it is where the line crosses the y-axis. If you know two points on a line, the formula m = ( y 2-y 1 ) / ( x 2-x 1 ) gives you the slope. Once you know a point and the slope, then the y-intercept can be found by substituting the coordinates of either point in the equation: y 1 = mx 1 + b , i.e., b = y 1-mx 1 . Alternatively, one can use the “point-slope” form of the equation of a straight line, which is: ( y-y 1 ) / ( x-x 1 ) = m . This...

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