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Ira Lisetskai [31]
3 years ago
12

Find the angle between u=<10,6> and v=<-3,14>.

Mathematics
1 answer:
Rus_ich [418]3 years ago
8 0
U
=
(
−
2
,
1
)
u
=
(
-
2
,
1
)
,
v
=
(
5
,
−
4
)
v
=
(
5
,
-
4
)
The equation for finding the angle between two vectors
θ
θ
states that the dot product of the two vectors equals the product of the magnitudes of the vectors and the cosine of the angle between them.
u
⋅
v
=
|
u
|
|
v
|
c
o
s
(
θ
)
u
⋅
v
=
|
u
|
|
v
|
c
o
s
(
θ
)
Solve the equation for
θ
θ
.
θ
=
a
r
c
⋅
c
o
s
(
u
⋅
v
|
u
|
|
v
|
)
θ
=
a
r
c
⋅
c
o
s
(
u
⋅
v
|
u
|
|
v
|
)
Find the dot product of the vectors.
Tap for more steps...
−
14
-
14
Find the magnitude of
u
u
.
Tap for more steps...
√
5
5
Find the magnitude of
v
v
.
Tap for more steps...
√
41
41
Substitute the values into the equation for the angle between the vectors.
θ
=
arccos
⎛
⎜
⎜
⎝
−
14
(
√
5
)
⋅
(
√
41
)
⎞
⎟
⎟
⎠
θ
=
arccos
(
-
14
(
5
)
⋅
(
41
)
)
Simplify.
Tap for more steps...
2.93049932
2.93049932
u
=
(
−
2
,
1
)
,
v
=
(
5
,
−
4
)
u
=
(
-
2
,
1
)
,
v
=
(
5
,
-
4
)
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#24, i am getting <img src="https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bcccc%7D1%260%260%26%5Cfrac%7B18%7D%7B7%7D%20
sveticcg [70]

It looks like you're talking about row-reducing an augmented matrix to solve the system of equations. Your answer is almost correct. The last row should read 0, 0, 1, 2/7.

The given system translates to

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 1 & -1 & 2 & 2 \\ 1 & 2 & -3 & 4 \end{array} \right]

Eliminate x from the last two rows by combining -2 (row 2) and row 1, and -2 (row 3) and row 1; that is,

(2x - 3y + z) - 2 (x - y + 2z) = 2 - 2 (2)

2x - 3y + z - 2x + 2y - 4z = 2 - 4

-y - 3z = -2

and

(2x - 3y + z) - 2 (x + 2y - 3z) = 2 - 2 (4)

2x - 3y + z - 2x - 4y + 6z = 2 - 8

-7y + 7z = -6

In augmented matrix form, this step yields

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 0 & -1 & -3 & -2 \\ 0 & -7 & 7 & -6 \end{array} \right]

I'll omit these details in the remaining steps.

Eliminate y from the last row by combining -7 (row 2) and row 3 :

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 0 & -1 & -3 & -2 \\ 0 & 0 & 28 & 8 \end{array} \right]

Multiply the last row by 1/28 :

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 0 & -1 & -3 & -2 \\ 0 & 0 & 1 & 2/7 \end{array} \right]

Eliminate z from the second row by combining 3 (row 3) and row 2 :

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 0 & -1 & 0 & -8/7 \\ 0 & 0 & 1 & 2/7 \end{array} \right]

Multiply the second row by -1 :

\left[ \begin{array}{ccc|c} 2 & -3 & 1 & 2 \\ 0 & 1 & 0 & 8/7 \\ 0 & 0 & 1 & 2/7 \end{array} \right]

Eliminate y and z from the first row by combining 3 (row 2), -1 (row 3), and row 1 :

\left[ \begin{array}{ccc|c} 2 & 0 & 0 & 36/7 \\ 0 & 1 & 0 & 8/7 \\ 0 & 0 & 1 & 2/7 \end{array} \right]

Multiply the first row by 1/2 :

\left[ \begin{array}{ccc|c} 1 & 0 & 0 & 18/7 \\ 0 & 1 & 0 & 8/7 \\ 0 & 0 & 1 & 2/7 \end{array} \right]

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3 : X

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