Introduction to Matrices (Edexcel A Level Further Maths: Core Pure)

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Naomi C

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Naomi C

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Introduction to Matrices

Matrices are a useful way to represent and manipulate data in order to model situations. The elements in a matrix can represent data, equations or systems and have many real-life applications.

What are matrices?

  • A matrix is a rectangular array of elements (numerical or algebraic) that are arranged in rows and columns
  • The order of a matrix is defined by the number of rows and columns that it has
    • The order of a matrix with m rows and n columns is m cross times n
  • A matrix bold italic A can be defined by bold italic A equals left parenthesis a subscript i j end subscript right parenthesis where i equals 1 comma space 2 comma space 3 comma space... comma space m and j equals 1 comma space 2 comma space 3 comma space... comma space n and a subscript i j end subscript refers to the element in row i, column j

u0sPLROY_1-7-1-ib-ai-hl-introduction-to-matrices-diagram

What type of matrices are there?

  • A column matrix (or column vector) is a matrix with a single columnn equals 1
  • A row matrix is a matrix with a single rowm equals 1
  • A square matrix is one in which the number of rows is equal to the number of columns, m equals n
  • Two matrices are equal when they are of the same order and their corresponding elements are equal, i.e. a subscript i j end subscript equals b subscript i j end subscriptfor all elements
  • A zero matrix, bold italic O, is a matrix in which all the elements are 0, e.g. bold italic O equals open parentheses table row 0 0 row 0 0 end table close parentheses
  • The identity matrix, bold italic I, is a square matrix in which all elements along the leading diagonal are 1 and the rest are 0, e.g. bold italic I equals open parentheses table row 1 0 row 0 1 end table close parentheses

What is the transpose of a matrix?

  • The transpose of matrix A is denoted as A
  • The transpose matrix is formed by interchanging the rows and columns

bold A to the power of T equals open parentheses table row cell a subscript 11 end cell cell a subscript 12 end cell cell a subscript 13 end cell row cell a subscript 21 end cell cell a subscript 22 end cell cell a subscript 23 end cell end table close parentheses space bold space bold space bold space bold A to the power of T equals open parentheses table row cell a subscript 11 end cell cell a subscript 21 end cell row cell a subscript 12 end cell cell a subscript 22 end cell row cell a subscript 13 end cell cell a subscript 23 end cell end table close parentheses

Examiner Tip

  • Make sure that you know how to enter and store a matrix on your calculator

Worked example

Let the matrix bold italic A equals open parentheses table row 5 cell negative 3 end cell 7 row cell negative 1 end cell 2 4 end table close parentheses

a)
Write down the order of bold italic A.

nIKgir9C_1-7-1-ib-ai-hl-introduction-to-matrices-we-1a-solution

b)
State the value of a subscript 2 comma 3 end subscript .

8~aiP3aD_1-7-1-ib-ai-hl-introduction-to-matrices-we-1b-solution

Basic Operations with Matrices

Just as with ordinary numbers, matrices can be added together and subtracted from one another, provided that they meet certain conditions.

How is addition and subtraction performed with matrices?

  • Two matrices of the same order can be added or subtracted
  • Only corresponding elements of the two matrices are added or subtracted
    • bold italic A plus-or-minus bold italic B equals left parenthesis a subscript i j end subscript right parenthesis plus-or-minus left parenthesis b subscript i j end subscript right parenthesis equals left parenthesis a subscript i j end subscript plus-or-minus b subscript i j end subscript right parenthesis
  • The resultant matrix is of the same order as the original matrices being added or subtracted

What are the properties of matrix addition and subtraction?

  • bold italic A plus bold italic B equals bold italic B plus bold italic A(commutative)
  • bold italic A plus left parenthesis bold italic B plus bold italic C right parenthesis equals left parenthesis bold italic A plus bold italic B right parenthesis plus bold italic C (associative)
  • bold italic A plus bold italic O equals bold italic A
  • bold italic O minus bold italic A equals negative bold italic A
  • bold italic A minus bold italic B equals bold italic A plus left parenthesis negative bold italic B right parenthesis

How do I multiply a matrix by a scalar?

  • Multiply each element in the matrix by the scalar value
    • k bold italic A equals left parenthesis k a subscript i j end subscript right parenthesis
  • The resultant matrix is of the same order as the original matrix
  • Multiplication by a negative scalar changes the sign of each element in the matrix

Worked example

Consider the matrices bold italic A equals open parentheses table row cell negative 4 end cell 2 row 7 3 row 1 cell negative 5 end cell end table close parenthesesbold italic B equals open parentheses table row 2 6 row 5 cell negative 9 end cell row cell negative 2 end cell cell negative 3 end cell end table close parentheses.

a)
Find bold italic A plus bold italic B.

1-7-2-ib-ai-hl-operations-with-matrices-we-1a-solution

b)
Find bold italic A minus bold italic B.

1-7-2-ib-ai-hl-operations-with-matrices-we-1b-solution

Multiplying Matrices

Can I always multiply a matrix by another matrix?

  • Not always - only if the dimensions of the matrices allow it
  • If A has order m cross times n and B has order q cross times p then you the matrix AB exists only if n equals q
  • The order of the matrix AB will be m cross times p
  • It is possible for AB to exist but BA not exist and vice versa
  • AB and BA both will exist if they are both square matrices of the same order 
    • This means the dimensions are the same n cross times n

How do I multiply a matrix by another matrix?

  • To multiply a matrix by another matrix, the number of columns in the first matrix must be equal to the number of rows in the second matrix
  • If the order of the first matrix is m cross times n and the order of the second matrix is n cross times p, then the order of the resultant matrix will be m cross times p
  • The product of two matrices is found by multiplying the corresponding elements in the row of the first matrix with the corresponding elements in the column of the second matrix and finding the sum to place in the resultant matrix
    • E.g. If bold italic A equals open square brackets table row a b c row d e f end table close square bracketsbold italic B equals open square brackets table row g h row i j row k l end table close square brackets
      • then bold italic A bold italic B equals open square brackets table row cell left parenthesis a g plus b i plus c k right parenthesis end cell cell left parenthesis a h plus b j plus c l right parenthesis end cell row cell left parenthesis d g plus e i plus f k right parenthesis end cell cell left parenthesis d h plus e j plus f l right parenthesis end cell end table close square brackets 
      • then  bold italic B bold italic A equals open square brackets table row cell left parenthesis g a plus h d right parenthesis end cell cell left parenthesis g b plus h e right parenthesis end cell cell left parenthesis g c plus h f right parenthesis end cell row cell left parenthesis i a plus j d right parenthesis end cell cell left parenthesis i b plus j e right parenthesis end cell cell left parenthesis i c plus j f right parenthesis end cell row cell left parenthesis k a plus l d right parenthesis end cell cell left parenthesis k b plus l e right parenthesis end cell cell left parenthesis k c plus l f right parenthesis end cell end table close square brackets

How do I square an expression involving matrices?

  • If an expression involving matrices is squared then you are multiplying the expression by itself, so write it out in bracket form first, e.g. left parenthesis A plus B right parenthesis squared equals left parenthesis A plus B right parenthesis left parenthesis A plus B right parenthesis
    • remember, the regular rules of algebra do not apply here and you cannot expand these brackets, instead, add together the matrices inside the brackets and then multiply the matrices together

What are the properties of matrix multiplication?

  • bold italic A bold italic B not equal to bold italic B bold italic A (non-commutative)
  • bold italic A left parenthesis bold italic B bold italic C right parenthesis equals left parenthesis bold italic A bold italic B right parenthesis bold italic C (associative)
  • bold italic A left parenthesis bold italic B plus bold italic C right parenthesis equals bold italic A bold italic B plus bold italic A bold italic C (distributive)
  • left parenthesis bold italic A plus bold italic B right parenthesis bold italic C equals bold italic A bold italic C plus bold italic B bold italic C (distributive)
  • bold italic A bold italic I equals bold italic I bold italic A equals bold italic A (identity law)
  • bold italic A bold italic O equals bold italic O bold italic A equals bold italic O, where bold italic O is a zero matrix
  • Powers of square matrices: bold italic A squared equals bold italic A bold italic A comma space bold italic A cubed equals bold italic A bold italic A bold italic A etc.

Worked example

Consider the matrices bold italic A equals open square brackets table row 4 2 cell negative 5 end cell row cell negative 3 end cell 8 1 row cell negative 1 end cell cell negative 2 end cell 2 end table close square brackets and bold italic B equals open square brackets table row 5 1 row cell negative 2 end cell 5 row 9 7 end table close square brackets .

a)
Find bold italic A bold italic B.

1-7-2-ib-ai-hl-operations-with-matrices-we-2a-solution

b)
Explain why you cannot find bold italic B bold italic A.

1-7-2-ib-ai-hl-operations-with-matrices-we-2b-solution

c)
Find bold italic A squared.

1-7-2-ib-ai-hl-operations-with-matrices-we-2c-solution

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Naomi C

Author: Naomi C

Expertise: Maths

Naomi graduated from Durham University in 2007 with a Masters degree in Civil Engineering. She has taught Mathematics in the UK, Malaysia and Switzerland covering GCSE, IGCSE, A-Level and IB. She particularly enjoys applying Mathematics to real life and endeavours to bring creativity to the content she creates.