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I am not sure how to explain this. I just know they have negative reciprocals because one one line will have a positive slope while the other negative.

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Two upvotes for 10,000 views. Interesting. – zz20s Apr 23 at 2:28
up vote 5 down vote accepted

Translate two lines so that their intersection is the origin and then take two vectors along each line, say $u=(1,k_1), v=(1,k_2)$. The two lines are perpendicular if and only if $u\perp v$, viz $$u\cdot v=1+k_1k_2=0$$ This explains why $k_1$ is the negative reciprocal of $k_2$.

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Since translation preserves angle we consider two perpendicular straight lines with slopes $m>0$ and $n$ through the origin. Feel free to draw a picture. Consider the triangles $(0,0)$, $(1,0)$, $(1,m)$ and $(0,0)$, $(-m,0)$, $(-m,1)$. Elementary geometry reveals immediately that both triangles are congruent, hence $n=-1/m$.

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This is the best answer, because the simplest. – TonyK Jan 8 '14 at 18:37

Draw any line (positive slope works best) other than a horizontal or a vertical. Choose any two points on the line, and let's say the rise between the two points is a and the run is b, so the slope of the line is a/b.

Now rotate your paper 90 degrees.

The same two points on the rotated line have rise b and run (-a), so the slope of the rotated line is -b/a.

Thus the product of the slopes, for the two perpendicular lines, is (a/b)*(-b/a) = -1.

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Given the equation y=mx + b, we can draw a triangle ABC with the vertical leg length m and the horizontal leg length 1. Next draw triangle ADE with DA perpendicular to AC.

enter image description here

ADE is congruent to ABC since angle DAE=angle CAB

We then have slope AC= rise/run = m/1 And slope DA = -1/m

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The triangles are congruent because of SAS, not because of equal angles. You don't know the angles are equal yet. – coolcheetah Aug 26 '15 at 16:57

Assuming experience with algebra without calculus background. So, I would suggest keeping to the idea that slope, $m$, is equal to "rise over run." Given a line with slope, lets say $\frac{a}{b}$, that means it rises $a$ in the $y$ direction for every $b$ it goes in the positive $x$ direction in the plane. (I would also encourage to always keep $b>0$ so we always go in the positive $x$ direction.) The way to make the slope the "most opposite" is to flip it and make it negative. Now, that is nowhere close to a proof, but I found one that only uses the fact that the Pythagorean theorem is true when you have a right angle and high school algebra.

Claim: If two lines in the plane, $f(x)=mx+b$ and $g(x)=nx+c$, are perpendicular, then $n=\frac{-1}{m}$.

Two lines $f(x)=mx+b$ and $g(x)=nx+c$ are not parallel, so they intersect. Assume that they do not intersect on the $y$-axis, i.e. $c \neq b$. Then the triangle formed by the graphs of these two lines and the $y$-axis is a right triangle if the Pythagorean theorem holds. WLOG, assume that $c>b$. The lengths of the side on the $y$-axis will be $c-b$. To find the other two sides, we do a little algebra. The intersection of these lines is $mx+b=nx+c$ and solving for $x$, we find that $x=\frac{c-b}{m-n}$. Thus, the point of intersection is


which we get by plugging in our $x$ for $f$ and $g$. To find the distance of each of the two legs of our triangle, we just use the distance formula, and find the the distance from

$$\left(\frac{c-b}{m-n},\frac{m(c-b)}{m-n}+b\right) \text{ to } (0,b) $$

is $\frac{(c-b)\sqrt{1+m^2}}{m-n}$. For the other side, we use the distance from

$$\left(\frac{c-b}{m-n},\frac{n(c-b)}{m-n}+c\right) \text{ to } (0,c) $$

which is $\frac{(c-b)\sqrt{1+n^2}}{m-n}$. Now, we set up the Pythagorean theorem, which we can use since the angle between the lines is right, and see that

$$ (c-b)^2 = \left[\frac{(c-b)\sqrt{1+m^2}}{m-n}\right]^2 + \left[\frac{(c-b)\sqrt{1+n^2}}{m-n}\right]^2 $$

Canceling the $(c-b)^2$ and multiplying by $(m-n)^2$ to both sides, we get

$$(m-n)^2 = 1+m^2 + 1+n^2$$ $$m^2 -2mn + n^2 = 2+m^2 +n^2 $$ Canceling and simplifying, we find that $n=\frac{-1}{m}$.

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Proof of the formula

You just need to know that

$\tan(x+\pi/2)=-\cot (x)$, and yes

$\cot (x)=1/\tan x$.

Also, you may reckon that in a triangle

$ \text{Sum of opposite interior angles = exterior angle}$

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Use $\LaTeX$ please. – user93957 Nov 28 '13 at 14:09
Sorry, I am not quite familiar with TEX format. Still Learning! – Shubham Nov 29 '13 at 17:17
Is the image being displayed? It is in BMP – Shubham Nov 29 '13 at 17:21
Yes, the image is displayed. For some basic information about writing math at this site see e.g. here, here, here and here. – user93957 Nov 29 '13 at 17:24
And write $tan$ as $\tan$ instead by adding a '\' to the function. For example cot: $cot$ \cot: $\cot$. – user93957 Nov 29 '13 at 17:26

This is one of the sad things happened in high school (mathematics).

Students are often asked to remember the formula m.M = –1 y heart without any explanation.

The proof, however, cannot be fully (and satisfactorily) explained because it requires the knowledge of another higher level topic called compound angle formulaes.

One of these formulas is tan (A – B) = (tan A – tan B) / (1 + tan A tan B). In case A – B = 90 degrees, tan A . tan B has to be –1. tan A happens to be m and tan B happens to be M (or vice versa). Hence, we have mM = –1.

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I don't think it requires trigonometry but you can demonstrate it as such. – oldrinb Oct 9 '13 at 4:13

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