Find the foot of the perpendicular drawn from (4, 1) upon the straight line 3x - 4y + 12 = 0.
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The line is
3x - 4y + 12 = 0 ...(i)
Any line perpendicular to (i) can be written as
4x + 3y + c = 0 ...(ii)
where c is an arbitrary constant.
Given that (ii) passes through the point (4, 1), then
4 (4) + 3 (1) + c = 0
or, 16 + 3 + c = 0
or, 19 + c = 0
or, c = - 19
Thus, the required line be
4x + 3y - 19 = 0 ...(ii)
Now, we solve (i) and (iii) to get the required foot of the perpendicular drawn from (4, 1)
3x - 4y + 12 = 0 ...(i) × 4
4x + 3y - 19 = 0 ...(iii) × 3
==>
12x - 16y + 48 = 0
12x + 9y - 57 = 0
On subtraction, we get
- 25y + 105 = 0
or, y = 21/5
Putting y = 21/5 in (ii), we get
4x + 3 (21/5) - 19 = 0
or, 4x = 19 - 63/5
or, 4x = 32/5
or, x = 8/5
So, the required foot of the perpendicular be
(8/5, 21/5)
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