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Worked Examples · Example 8

Q.Find the equation of the line, which makes intercepts −3-3 and 22 on the x- and y-axes respectively.

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The intercept form of a line is xa+yb=1\frac{x}{a} + \frac{y}{b} = 1, where aa and bb are the x- and y-intercepts. Substituting a=−3a = -3 and b=2b = 2 gives the equation 2x−3y+6=02x - 3y + 6 = 0.

The key idea here is the intercept form of a straight line. When a line cuts the x-axis at a point (a,0)(a, 0) and the y-axis at (0,b)(0, b), the numbers aa and bb are called the x-intercept and y-intercept respectively. The beauty of this form is that it directly uses these two pieces of information to write the equation — no slope calculation needed.

Why does this work? If a line passes through (a,0)(a, 0) and (0,b)(0, b), its slope is b−00−a=−ba\frac{b - 0}{0 - a} = -\frac{b}{a}. Using the point-slope form with point (0,b)(0, b) gives y−b=−ba(x−0)y - b = -\frac{b}{a}(x - 0), which simplifies to xa+yb=1\frac{x}{a} + \frac{y}{b} = 1. So the intercept form is just a tidy rearrangement of the standard two-point form.

xa+yb=1\frac{x}{a} + \frac{y}{b} = 1

where aa is the x-intercept and bb is the y-intercept.

Now, let’s apply it step by step.

  1. Identify the intercepts. The problem states: intercepts −3-3 on the x-axis and 22 on the y-axis. So a=−3a = -3 and b=2b = 2. Note that the negative sign on the x-intercept means the line crosses the x-axis to the left of the origin.

  2. Plug into the intercept form.

x−3+y2=1\frac{x}{-3} + \frac{y}{2} = 1

  1. Simplify the equation. Multiply both sides by the least common multiple of the denominators, which is 66, to clear fractions:

6⋅x−3+6⋅y2=6⋅16 \cdot \frac{x}{-3} + 6 \cdot \frac{y}{2} = 6 \cdot 1

−2x+3y=6-2x + 3y = 6 …

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