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2) ↑↑↑↑
3) =
5) Given: the coordinate system
the vector modulus = 5
= 12
Find: OA
Solution:
OA== 13
OA = = 13
Answer: OA = 13
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Let and be the given vectors.
1) From the point O we draw the vectors = and =
2) If ↑↓- are oppositely directed vectors, we see that the rays OA and OB form an angle AOB
3) If ↑↑- are co-directional vectors, we see that the angle between the vectors and is equal to 0°.
The angle between two vectors and is designated as:
Definition:
Two vectors are called perpendicular if the angle between them is 90°.
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ABCD - is the square
AC ∩ BD = O
Find: the angles between the vectors BAC, DAB = ?
Calculation:
a) Since AC - is the diagonal of the square, we see that it divides the angle A in half. Then the angle between the vectors = 45°
b) Since ABCD - is a square, we see that the degree measure of the angle between the vectors = = 90°, i.e. right angle.
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ABCD - is the rhombus
BD = AB; AC ∩ BD = 0
Compute: the angle formed by the vectors
and , and , and = ?
Solution:
a) By the definition of the rhombus ΔABD - is equilateral (AB = AD = BD).
Hence, all the angles in the triangle are equal to 60°. Then the angle between the vectors = 60°
b) Since the vectors ↑↑are co-directional, the angle between the vectors = 0°
c) Since the vectors ↑↓ - are oppositely directed, we see that the angle between the vectors = 180°
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Definition:
A scalar product of two vectors (formula 1) is the product of the lengths of these vectors by the cosine of the angle between them.
Designation: or
= *cos (a,b) (1)
From the formula of the scalar product of vectors having used the cosine of the angle (1) it follows that:
1) the scalar product of vectors is greater than zero if the angle between vectors is less than 90°, i.e.
>0 if <90°
the scalar product of the vectors is less than zero if the angle between the vectors is greater than 90°, i.e.
<0 if >90°
2) If ↑↑ are co-directional vectors, we see that the angle between the vectors is equal to zero degrees, i.e. =0° =
3) If are the perpendicular vectors and =90° Cos 90° = 0, then = 0
The converse is also true, i.e. if = 0
Conclusion: = 0
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Given:
the vectors
=2
=3
the angle α = 90°
Find: scalar product of vectors
Solution:
Using the formula of the scalar multiplication of vectors by the cosine of the angle, we obtain
= •• Cos 90° = 2 • 3 • 0 = 0
Answer: = 0
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***
Given:
the triangle ΔABC is equilateral
Find: scalar product of vectors 1) 2)
Solution: In an equilateral triangle, all angles are equal to 60°.
1) = •Cos () = • =
2) = •Cos (120°) = -
***
Given:
the vectors
=2; =3
1) the angle α = 45°
2) α = 135°
Find: scalar or inner product of vectors
Solution:
1) = •Cos 45° = 2 • 3 • = 3
2) = •Cos 135° = 2 • 3 • = -3
Answer: 1) 3; 2) -3
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Given:
AB = a
the height BD
Find: scalar multiplication of vectors
1)
2) 3)
Solution:
1) = •Cos 120° = • (-Cos 60°) = -
2) since the vectors are perpendicular BDAC = 0
3) = =
Answer:1) -; 2) 0 ; 3)
Given:
BD ∩ AC = 0
BD = AB
1) ;
2) ;
Find: the value of the angle between the vectors
1) ; 2)
Solution:
1) Consider the triangle ΔABC. This trianle is isosceles, since AB=BD.
Knowing that in the rhombus all sides are equal, we get ΔABD - is equilateral.
Then DAB =BDA = 60°
By the property of the rhombus it follows that ADC = 120°
Then the angle between the vectors =120°
2) Since the sides are parallel and the vectors are co-directed:
BA || CD and ↑↑ , then the vectors are parallel ||, therefore the vectors are equal =.
Consider a triangle ΔCBD is isosceles, since the two sides are equal: BD=BC.
By the definition of the rhombus, the triangle ΔCBD - is equilateral.
Hence, the angle BDC = 60°
By the property of the rhombus, the angle ADC = 120°.
Then the angle between the vectors is =120°.
Answer: 1) =120°; 2) =120°.
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Theorem:
If two vectors have coordinates {x1; y1}; { x2; y2}, then the scalar product of two vectors (formula 2) is the product of their coordinates:
Proof:
Case 1.
Case 2.
If the vectors and - are noncollinear.
We draw the vectors from an arbitrary point O.
Consider the triangle ΔOBA.
It is known that the cosine formula
c2 = a2 + b2 - 2ab • Cos α, we obtain the equality
AB2 = OB2 + OA2 - 2 • OB • OA • Cos α (3)
Using the values (*) = ; = ; = ; and also OA = ||; OB = || ; AB = ||, substituting the values of (*) in the equality (3), we obtain
||2 = ||2 + ||2 - 2 (4)
Using the formula to calculate the length of the vector having used its coordinates, we obtain
=; =.
Since = {x2 - x1; y2 – y1}, then using the formula to calculate the distance between two points
, we obtain
|| = .
Then it follows from the equality (4) that
(x2 - x1)2 + (y2 – y1)2 = x22 + y22 + x12 + y12 - 2
x22 -2 x2 x1 + x12 + y22 – 2 y2y1 + y12 = x22 + y22 + x12 + y12 - 2
-2 x2 x1– 2 y2y1 = - 2
= x2 x1 + y2 y1
***
Consequences:
1) If the vectors are perpendicular, i.e.
{x1; y1}{ x2; y2} x1 x2 + y1 y2 = 0
2) By the definition of the scalar or inner product of two vectors (formula 1)
= •• Cos α
Cos α =
The formula to calculate the cosine of the angle having used the coordinates of the vectors:
To calculate the sine and the tangent of the angle between vectors by the cosine of an angle it’s used the reduction formulas and trigonometric functions.
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If {; -1}; {2; 3}, then = 0.5 + (-3) = -2.5
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If {x; -1}; {3; 2} and the vectors are perpendicular , then = 3x - 2 0 = 3x - 2 2 = 3x x =
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Given:
the coordinates of the points
A(2;8), B(-1;5), C(3;1)
Solution:
Since each coordinate of the vector is equal to the difference of the corresponding coordinates of its end and the beginning
{b1 – a1; b2 – a2}, then
= {} = {}
= {} = {}
Using the formula to find the angles having used the coordinates of the vectors
Cos A =, we get
Cos A = ===
Answer: Cos A =
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Given:
the angle between the vectors is equal to = =60° ,
magnitudes or lengths of vectors || = 1, || = || = 2
Find: the product of vectors ()= ?
Solution:
()=+= ||•||•Cos 60° + ||•||•Cos 60° = 1 + 2 = 3
Answer: ()= 3
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Given:
=, =
the magnitude or the length of the vectors ||=||=1
- are perpendicular vectors
Find: product of vectors = ?
Solution:
= ()•() = 32 + 12 - 2- 82 =
= 32 + 10 - 82 = 3||2 + 0 - 8||2 = -5.
Answer: = -5.
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Given:
{1.5 ; 2}, {4 ; -0.5}
Find: multiplication of vectors = ?
Solution:
= x1 x2 + y1 y2 = 6 + (-1) = 5
Answer: = 5.
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Given:
{0 ; -3}, {5 ; x}
are perpendicular vectors
Find: multiplication or product of vectors = ?
Solution:
= x1 x2 + y1 y2
0 = 0 + (-3x)
3x = 0
x = 0
Answer: under x=0, .
***
Given:
the coordinates of the points
A(2;8), B(-1;5), C(3;1)
Find: the cosine of the angle of vectors
2) Cos C = ?
Solution:
1)
Since each coordinate of the vector is equal to the difference of the corresponding coordinates of its end and the beginning
{b1 – a1; b2 – a2}, then
= {} = {}
= {} = {}
Using the formula to find the angles having used the given vector coordinates
Cos B = , we get
Cos B = == 0
2)
= {} = {}
= {} = {}
Cos C = ===
Answer: Cos B =0, Cos C =
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Given:
, where i and j are coordinate vectors
Find: the length or the magnitude of the vector || = ?
Solution:
Let us find the coordinates of the vector .
{3; -4}
Since the length of the vector is equal to the square root of the sum of the squares of its coordinates || = , then we get
|| = = = 5.
Answer: || = 5.
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Given:
ABCD is the rhombus
AB =, AD =
ACBD or =0
Proof:
Since the figure ABCD is the rhombus and the parallelogram, then the vectors of the parallelogram ,
= -
= (+) (-) = - 2 + 2 - = 2 -2 = =||2 -||2 = 0. Therefore, the angle between the vectors = 90°. Hence, the diagonals of the rhombus at the intersection point are perpendicular, i.e. ACBD.
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Given:
the triangle ΔABC - is isosceles
AM is the median
Prove:
1) 4AM2 = AB2 + AC2 + 2AB • AC • Cos A
Proof:
1) Since the point M is the midpoint of the side BC, we see that
2=
Therefore, (2) • (2) = ()() =
= AB2 + 2AB + 2AB • AC • Cos A + AC2 = AB2 + AC2 + 2AB • AC • Cos A
We get 4AM2 = AB2 + AC2 + 2AB • AC • Cos A
2) By the formula obtained above, it follows that
4CH2 = AC2 + BC2 + 2AC • BC • Cos C
Since the triangle ΔABC is isosceles, we see that AB = BC, A = C Cos A = Cos C
We get that 4CH2 = AC2 + BC2(=AB2) + 2AC • BC(=AB) • Cos C (= Cos A)
4CH2 = AC2 + AB2 + 2AC • AB • Cos A
4CH2 = 4AM2
=
2CH = 2AM | : 2
CH = AM
***
Given:
ABCD is the convex quadrilateral
BD = d1 and AC = d2 are the diagonals
d1 ∩ d2 = O is the intersection point of the diagonals
Prove:
The area of the quadrilateral is equal to half the product of the diagonals by the sine of the acute angle between them
SABCD= d1 • d2 • Sin α
Proof:
The area of a quadrilateral is the sum of the areas of four triangles.
SABCD= S1 + S2 + S3 + S4 , where
S1 = SΔAOB ; S2 = SΔCOB ; S3 = SΔCOD ; S4 = SΔAOD
S1 = BO • OA • Sin α
S2 = BO • OC • Sin (180° - α) = BO • OC • Sin α
S3 = CO • OD • Sin α
S4 = AO • OD • Sin (180° - α) = AO • OD • Sin α
Adding S1 + S2 + S3 + S4, we obtain
SABCD= BO • Sin α (OA+OC) +
+ OD • Sin α (CO+OA)
Since OA+OC = AC, CO+OA = AC, BO + OD = BD we see that
SABCD=BO • AC • Sin α +OD • AC • Sin α =BD • AC • Sin α
The formula of the area of a convex quadrilateral:
SABCD= d1 • d2 • Sin α
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Given:
two vectors form an angle α = 150°,
the magnitudes or the lengths of vectors || = 2 , || = 2
Solution:
BC2 = AB2 + AC2 - 2 AB • AC • Cos 150°
BC2 = 48 + 4 - 2 • 4• 2 • (-) = 52 + 24 = 76
BC = = 2
Answer: BC = |2-| = 2
***
Given:
The angles B = 45°, C = 70°
a=24.6
Find: A - the angle in degrees, the sides b, c
Solution:
A = 180° - (45° + 70°) = 75°
Using the law of sines
, we obtain the expression
b = ≈ 19.2
c = ≈
≈ 25.5
Answer: A = 75°; b ≈ 19.2; c ≈ 25.5.
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Given:
the lengths of vectors || = 5, || = 8,
Find: the value of vectors
1) ||= ?
2) ||= ?
Solution: by the law of cosines
1)
AC2 = AB2 + BC2 - 2AB • BC • Cos 120°
AC2 = 25 + 64 - 80 • (- 0.5) = 129
2) BC2 = AB2 + AC2 - 2AB • AC • Cos 60°
BC2 = 89 - 80 • 0.5 = 49
BC = ±, but BC = - 7 does not satisfy the solution of the problem. Therefore, BC = 7.
Answer: || =; || = 7.
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