Graph of Functions
In 2-dimension plane, right angles can be formed by two intersected lines which the product of their slopes equals to −1. More precisely, defining two linear functions: y1 = a1x + b1 and y2 = a2x + b2, the graph of the functions will be perpendicular and will make four right angles where the lines intersect if and only if a1a2 = −1. However, this method cannot be used if the slope is zero or undefined (the line is parallel to an axis).
For another method, let the two linear functions: a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0. The lines will be perpendicular if and only if a1a2 + b1b2 = 0. This method is simplified from the dot product (or generally, inner product) of vectors. In particular, two vectors are considered orthogonal if their inner product is zero.
Read more about this topic: Perpendicular
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