The distance formula between two points, if a(x 1, x2)b(y 1, y2),

Then the absolute value of the module of ab =

Root number [(x1-y1) 2+(x2-y2) 2]

For the length formula of vector, if the modulus of A =(a 1, a2), then the absolute value of the modulus of A = root sign (a12+a2 2).

The coordinate formula of the included angle between two vectors, if a(a 1, a2)b(b 1, b2),

Then cos

(that is, the product of vectors divided by the product of modules)

So, because

(a 1b 1+a2b2)/[ radical symbol (a 1 2+a2 2) * radical symbol (b 1 2+b2 2)]

Let a(x 1, x2)b(y 1, y2),

Absolute value of ab =|a*b|=|

x 1y 1+x2y2

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