electromagnetics question ,plsss help me guys?

Question

given two vectors rA= -ax-3ay-4az and rB=2ax+2ay+2az

find rab

and the answer is 

RAB =(2+1)ax +(2+3)ay +(2+4)az
RAB= 3ax+5ay+6az

now my question is about the signs..why the two vectors added with positive sign..isnt it that it should rab=(2-1)ax+ (2-3)ay+(2-4)az

pls help me how to add vectors..wheter from positive to negative..please explain it to me guysss.plsss

? · Accepted Answer

"(2-1)ax+ (2-3)ay+(2-4)az"  is not RAB this is RA+RB
RAB will be RB-RA and that would change all signs of RA
RB-RA= 2ax+2ay+2az - ( -ax-3ay-4az)= 2ax+2ay+2az+ ax+3ay+4az= (2+1)ax +(2+3)ay +(2+4)az

Both vector RA and RB are originating from origin, where RAB means the vector connecting  head of RA to the head of RB, which will be RB-RA.

If you need more explanation, you may ask.

? · Answer

Rab=Ra-Rb.  You need to observe the signs.  Try again.

electromagnetics question ,plsss help me guys?

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