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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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7.2 Vectors and scalars: basic concepts 229

B

B

AB

DB

BA

BD

A

DA

D

A

AD

D

Figure7.11

ThevectorsABand → DA.

Figure7.12

ThevectorsBAand → AD.

Solution We have

hence

b = → AD

−b = → DA

Then

a−b=a+(−b)

= → AB + → DA

= → DA + → AB by commutativity

DAand → ABareshown inFigure 7.11. Their sum isgiven by the triangle law, thatis

Similarly,

hence

a−b= → DA + → AB = → DB

a = → AB

−a = → BA

Then

b−a=b+(−a)

= → AD + → BA

= → BA + → AD

BAand → ADareshowninFigure7.12.Againtheirsumisgivenbythetrianglelaw,thatis

b−a= → BA + → AD = → BD

Example7.3 Referring to Figure 7.13, if r 1

and r 2

are as shown, find the vector b = → QP represented

by the third sideof the triangle OPQ.

Solution FromFigure 7.13 we note fromthe triangle law that

QP = → QO + → OP = → OP + → QO by commutativity.

ButQO = −r 2

andQP → = b, and so

b=r 1

−r 2

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