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The point slope form of a linear equation is written below:
y-y1 = m(x-x1)
m is the slope of the equation and x1 and y1 are the coordinates for a point on the equation. So if we have a slope and the coordinates for a point then we can plug them into the above equation to get the point slope form of the line. So if we are looking at the equation for a horizontal line that passes through (-4,-1), we are already given a point to plug into the equation. Slope is rise over run, and a horizontal line has no rise (it has a constant value of y for any x) so its slope is 0. We can now plug these values into the equation:
y-(-1) = 0(x-(-4))
y+1 = 0(x+4)
y+1 = 0
Taking note that slope of a horizontal line is zero, we can use the slope-intercept form y = mx + b; where m = slope, x = abscissa, and y = ordinate.
Plugging-in y = -1, m = 0 and x = -4 on y = mx + b:
- 1 = 0(-4) + b
- 1 = 0 + b
- 1 = b or b = -1.
Plugging-in m = 0 and b = -1 on y = mx + b:
y = 0x - 1
==> y = -1 or y + 1 = 0.
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