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# Help in geometry

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1 THEOREM 8-5Two nonvertical lines are perpendicular if, and only if, the slope of one is the negative reciprocal of the slope of the other line: .

To find the negative reciprocal of a number, first invert the number then change the sign. For example, to find the negative reciprocal of 4/3, you would first invert the number to get 3/4 and then change the sign (in this case from positive to negative) to get -3/4.

Two nonvertical lines are perpendicular if, and only if, the slope of one is the negative reciprocal of the slope of the other line: can any one help me under stand this THEOREM

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Sep 14, 2019

#1
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Not a tough as it looks

Let's suppose that  the slope of  some line  =  a/b

This theorem says that   another line will be perpendicular  if it  has  a negative reciprocal slope

The negative reciprocal of a/b   = -b/a

Farther.....if we multiply these two slope together and the result  = -1.....the lines are perpendicular

So

(a/b) (-b/a)  =

- (a/b) (b/a)  =

-  (ab) / (ba)  =

- (ab) / (ab)  =

- 1        ,,,,so....we see that they are truly perpendicular lines  !!!!!   Sep 14, 2019