# Difference between revisions of "12-240/Classnotes for Thursday September 20"

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In this class, the professor completes the lecture about complex number and then introduces vector space.

## Complex number

Definition:

C={(a,b): a, b $\in\!\,$ R}

1 ( of C) = (1,0); 0 ( of C)= (0,0)

i=(0,1)

(a,b)+(c,d)=(a+c,b+d); (a,b)x(c,d)=(ac-bd,ad+bc)

i^2=-1