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Solution<br />

Consider the following diagram.<br />

x<br />

x + y<br />

–y<br />

x – y<br />

y<br />

Since<br />

0x ! y ! 0 0x ! y ! 0 ,<br />

x<br />

0 x ! y ! 0 2 0 x ! y ! 0<br />

2<br />

! !<br />

and<br />

Therefore,<br />

1x ! 0 x ! <br />

y ! y ! 0 2 1x ! y ! 2 #<br />

! ! 0x y 0 2 1x ! y ! 2 #<br />

! ! 1x y 2<br />

1x y 2<br />

! !<br />

2 #<br />

! ! 1x y 2 #<br />

! ! 1x y 2 1x y 2<br />

0 x ! 0 2 2x ! # y<br />

! 0 y<br />

! 0 2 0 x ! 0 2 2x ! # y<br />

! 0 y<br />

! 0<br />

2<br />

(Squaring both sides)<br />

(Magnitudes<br />

property)<br />

(Multiplying out)<br />

So, 4x ! #<br />

! y 0 and x! #<br />

! y 0<br />

Thus, the two required vectors are shown to be perpendicular. (Geometrically,<br />

this means that if diagonals in a parallelogram are equal in length, then the sides<br />

must be perpendicular. In actuality, the parallelogram is a rectangle.)<br />

In this section, we dealt with the dot product and its geometric properties. In the<br />

next section, we will illustrate these same ideas with algebraic vectors.<br />

IN SUMMARY<br />

Key Idea<br />

• The dot product between two geometric vectors and is a scalar quantity<br />

defined as a ! !<br />

#<br />

! ! a ! b !<br />

b 0a 0 @ b @ cos u , where u is the angle between the two vectors.<br />

Need to Know<br />

• If<br />

, then<br />

• If , then a ! a ! !<br />

0° u 6 90° # b 7 0<br />

!<br />

u 90° # b 0 SUMMARY<br />

• If<br />

, then<br />

• a ! !<br />

# ! a ! !<br />

90° 6 u 180° # b 6 0<br />

b b #<br />

! a<br />

• a ! !<br />

#<br />

! !<br />

1b c 2 a! # b a! #<br />

! c<br />

• a ! #<br />

! ! a 0a 0<br />

• ! ! ! 2<br />

!<br />

and k ! !<br />

i # i 1, j # j 1, # k 1<br />

• 1ka ! 2 # b<br />

!<br />

a! # 1kb<br />

!<br />

2 k1a! # b<br />

!<br />

2<br />

376 7.3 THE DOT PRODUCT OF TWO GEOMETRIC VECTORS<br />

NEL

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