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y = 9 is a horizontal line with slope 0. Slope = (y2 - y1)/(x2-x1), and y is constant so that numerator is always 0. The y-intercept is where the line crosses the y-axis when x=0. Since y is always 9 in this case, that is the y-intercept.
If the numerator is 0, the fraction equals 0, because there is no part of the whole.
If the numerator is 0 AND the denominator is not 0, then the ratio is 0. And cosine of 0 is 1.
Negative reciprocals. That is, if one line has slope m (m ≠0), then the perpendicular to it has slope -1/m. If m = 0, the slope of the perpendicular is not defined - the line is of the form x = k.
If the original slope is 0, then the opposite reciprocal of 0 is still 0. Oops! Reciprocal of 0 (n/0) is undefined! A vertical line has undefined slope, the horizontal line has slope of 0.