The Matrix of a Linear Transformation

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Section 1.9 (Through Theorem 10) The Matrix of a Linear Transformation. Identity Matrix In is an n n matrix with 1's on the main left to right diagonal and 0's ...
Section 1.9 (Through Theorem 10) The Matrix of a Linear Transformation Identity Matrix I n is an n × n matrix with 1’s on the main left to right diagonal and 0’s elsewhere. The ith column of I n is labeled e i . EXAMPLE: 1 0 0 I3 =

e1 e2 e3

=

0 1 0 0 0 1

Note that

I3x =

= ____

1 0 0

x1

0 1 0

x2

0 0 1

x3

+ ____

+ ____

= ____.

In general, for x in R n , I n x = ___

From Section 1.8, if T : R n → R m is a linear transformation, then Tcu + dv =cTu +dTv .

Generalized Result: Tc 1 v 1 + ⋯ + c p v p =c 1 Tv 1  + ⋯ + c p Tv p .

1

1 0

EXAMPLE: The columns of I 2 =

1

are e 1 =

0 1 2 3 linear transformation from R to R where

and e 2 =

0

2 Te 1  =

x1

1

. Suppose T is a

5

−3

and Te 2  =

4

Compute Tx  for any x =

0

0

.

1

.

x2

Solution: A vector x in R 2 can be written as x1

= _____

x2

1

0

+ _____

0

= _____e 1 + _____e 2

1

Then Tx  = Tx 1 e 1 + x 2 e 2  = _____Te 1  + _____Te 2  2 = _____

−3

5 + _____

4

0

=

.

1

Note that x1

Tx  =

x2

.

So Tx  =

Te 1  Te 2 

To get A, replace the identity matrix

x = Ax

e1 e2

with

Te 2  Te 2 

.

2

Theorem 10 Let T : R n → R m be a linear transformation. Then there exists a unique matrix A such that Tx = Ax for all x in R n . In fact, A is the m × n matrix whose jth column is the vector Te j , where e j is the jth column of the identity matrix in R n . A = Te 1 

Te 2 



Te n 

↑ standard matrix for the linear transformation T

? ? EXAMPLE:

x1

? ?

x2

? ?

x 1 − 2x 2 =

4x 1 3x 1 + 2x 2

Solution: ? ? ? ?

= standard matrix of the linear transformation T

? ? ? ? ? ?

=

Te 1  Te 2 

=

(fill-in)

? ?

3

EXAMPLE: Find the standard matrix of the linear transformation T : R 2 → R 2 which rotates a point about the origin through an angle of π4 radians (counterclockwise). 1

1

−1

1

−1

1

−1

−1

Te 1  =

Te 2  =



A=

4

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