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This chapter covers principles of matrices. After completing this chapter students should be able to: complete matrix operations; solve linear systems using Gauss-Jordan method; Solve linear systems using the matrix inverse method and complete application problems.

Introduction to matrices

A vendor sells hot dogs and corn dogs at three different locations. His total sales (in hundreds) for January and February from the three locations are given in the table below.

January February
Hot Dogs Corn Dogs Hot Dogs Corn Dogs
Place I 10 8 8 7
Place II 8 6 6 7
Place III 6 4 6 5

Represent these tables as 3 × 2 size 12{3 times 2} {} matrices J size 12{J} {} and F size 12{F} {} , and answer [link] , [link] , [link] , and [link] . problems 1 - 4.

Determine total sales for the two months, that is, find J + F size 12{J+F} {} .

18 15 14 13 12 9 size 12{ left [ matrix { "18" {} # "15" {} ##"14" {} # "13" {} ## "12" {} # 9{}} right ]} {}
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Find the difference in sales, J F size 12{J - F} {} .

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If hot dogs sell for $3 and corn dogs for $2, find the revenue from the sale of hot dogs and corn dogs. Hint: Let P size 12{P} {} be a 2 × 1 size 12{2 times 1} {} matrix. Find J + F P size 12{ left (J+F right )P} {} .

84 68 54 size 12{ left [ matrix { "84" {} ##"68" {} ## "54"} right ]} {}
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If March sales will be up from February by 10%, 15%, and 20% at Place I, Place II, and Place III, respectively, find the expected number of hot dogs, and corn dogs to be sold in March. Hint: Let R size 12{R} {} be a 1 × 3 size 12{1 times 3} {} matrix with entries 1.10, 1.15, and 1.20. Find RF size 12{ ital "RF"} {} .

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Determine the sums and products in the next 5 problems. Given the matrices A size 12{A} {} , B size 12{B} {} , C size 12{C} {} , and D size 12{D} {} as follows:

A = 3 6 1 0 1 3 2 4 1 size 12{A= left [ matrix { 3 {} # 6 {} # 1 {} ##0 {} # 1 {} # 3 {} ## 2 {} # 4 {} # 1{}} right ]} {} B = 1 1 2 1 4 2 3 1 1 size 12{B= left [ matrix { 1 {} # - 1 {} # 2 {} ##1 {} # 4 {} # 2 {} ## 3 {} # 1 {} # 1{}} right ]} {} C = 1 2 3 size 12{C= left [ matrix { 1 {} ##2 {} ## 3} right ]} {} D = 2 3 2 size 12{D= left [ matrix { 2 {} # 3 {} # 2{}} right ]} {}

3A 2B size 12{3A - 2B} {}
7 20 1 2 5 5 0 10 1 size 12{ left [ matrix { 7 {} # "20" {} # - 1 {} ##- 2 {} # - 5 {} # 5 {} ## 0 {} # "10" {} # 1{}} right ]} {}
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AB + BA size 12{ ital "AB"+ ital "BA"} {}
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A 2 size 12{A2} {}
11 28 22 6 13 6 8 20 15 size 12{ left [ matrix { "11" {} # "28" {} # "22" {} ##6 {} # "13" {} # 6 {} ## 8 {} # "20" {} # "15"{}} right ]} {}
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2 BC size 12{2 ital "BC"} {}
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2 CD + 3 AB size 12{2 ital "CD"+3 ital "AB"} {}
40 72 61 38 33 23 39 63 51 size 12{ left [ matrix { "40" {} # "72" {} # "61" {} ##"38" {} # "33" {} # "23" {} ## "39" {} # "63" {} # "51"{}} right ]} {}
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Let E = m n p q size 12{E= left [ matrix { m {} # n {} ##p {} # q{} } right ]} {} and F = a b c d size 12{F= left [ matrix { a {} # b {} ##c {} # d{} } right ]} {} , find EF size 12{ ital "EF"} {} .

ma + nc mb + nd pa + qc pb + qd size 12{ left [ matrix { ital "ma"+ ital "nc" {} # ital "mb"+ ital "nd" {} ##ital "pa"+ ital "qc" {} # ital "pb"+ ital "qd"{} } right ]} {}
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Let G = 3 6 1 0 1 3 2 4 1 size 12{G= left [ matrix { 3 {} # 6 {} # 1 {} ##0 {} # 1 {} # 3 {} ## 2 {} # 4 {} # 1{}} right ]} {} H = x y z size 12{H= left [ matrix { x {} ##y {} ## z} right ]} {} , find GH size 12{ ital "GH"} {} .

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Express the following systems as AX = B size 12{ ital "AX"=B} {} , where A size 12{A} {} , X size 12{X} {} , and B size 12{B} {} are matrices.

4x 5y = 6 size 12{4x - 5y=6} {}
5x 6y = 7 size 12{5x - 6y=7} {}
4 5 5 6 x y = 6 7 size 12{ left [ matrix { 4 {} # - 5 {} ##5 {} # - 6{} } right ]left [ matrix { x {} ##y } right ]= left [ matrix { 6 {} ##7 } right ]} {}
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x 2y + 2z = 3 size 12{x - 2y+2z=3} {}
x 3y + 4z = 7 size 12{x - 3y+4z=7} {}
x 2y 3z = 12 size 12{x - 2y - 3z= - "12"} {}
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2x + 3z = 17 size 12{2x+3z="17"} {}
3x 2y = 10 size 12{3x - 2y="10"} {}
5y + 2z = 11 size 12{5y+2z="11"} {}
2 0 3 3 2 0 0 5 2 x y z = 17 10 11 size 12{ left [ matrix { 2 {} # 0 {} # 3 {} ##3 {} # - 2 {} # 0 {} ## 0 {} # 5 {} # 2{}} right ] left [ matrix {x {} ## y {} ##z } right ]= left [ matrix { "17" {} ##"10" {} ## "11"} right ]} {}
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x + 2y + 3z + 2w = 14 x 2y z = 5 y 2z + 4w = 9 x + 3z + 3w = 15 size 12{ matrix { x {} # +{} {} # 2y {} # +{} {} # 3z {} # +{} {} # 2w {} # ={} {} # "14" {} ##x {} # - {} {} # 2y {} # - {} {} # z {} # {} # {} # ={} {} # - 5 {} ## y {} # - {} {} # 2z {} # {} # {} # +{} {} # 4w {} # ={} {} # 9 {} ##x {} # +{} {} # 3z {} # {} # {} # +{} {} # 3w {} # ={} {} # "15"{} } } {}
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Systems of linear equations

Solve the following by the Gauss-Jordan Method. Show all work.

x + 3y = 1 size 12{x+3y=1} {}
2x 5y = 13 size 12{2x - 5y="13"} {}

(4,-1)

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x y z = 1 size 12{x - y - z= - 1} {}
x 3y + 2z = 7 size 12{x - 3y+2z=7} {}
2x y + z = 3 size 12{2x - y+z=3} {}
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x + 2y + 3z = 9 size 12{x+2y+3z=9} {}
3x + 4y + z = 5 size 12{3x+4y+z=5} {}
2x y + 2z = 11 size 12{2x - y+2z="11"} {}

(2, -1, 3)

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x + 2y = 0 size 12{x+2y=0} {}
y + z = 3 size 12{y+z=3} {}
x + 3z = 14 size 12{x+3z="14"} {}
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Two apples and four bananas cost $2.00 and three apples and five bananas cost $2.70. Find the price of each.

(0.4, 0.3)

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A bowl of corn flakes, a cup of milk, and an egg provide 16 grams of protein. A cup of milk and two eggs provide 21 grams of protein, and two bowls of corn flakes with two cups of milk provide 16 grams of protein. How much protein is provided by one unit of each of these three foods.

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x + 2y = 10 size 12{x+2y="10"} {}
y + z = 5 size 12{y+z=5} {}
z + w = 3 size 12{z+w=3} {}
x + w = 5 size 12{x+w=5} {}

(4, 3, 2, 1)

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x + w = 6 size 12{x+w=6} {}
2x + y + w = 16 size 12{2x+y+w="16"} {}
x 2z = 0 size 12{x - 2z=0} {}
z + w = 5 size 12{z+w=5} {}
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Systems of linear equations – special cases

Solve the following inconsistent or dependent systems by using the Gauss-Jordan method.

2x + 6y = 8 size 12{2x+6y=8} {}
x + 3y = 4 size 12{x+3y=4} {}
( 4 3t size 12{4 - 3t} {} , t size 12{t} {} )
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The sum of the digits of a two digit number is 9. The sum of the number and the number obtained by interchanging the digits is 99. Find the number.

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2x y = 10 size 12{2x - y="10"} {}
4x + 2y = 15 size 12{ - 4x+2y="15"} {}

Inconsistent system, no solution

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x + y + z = 6 size 12{x+y+z=6} {}
3x + 2y + z = 14 size 12{3x+2y+z="14"} {}
4x + 3y + 2z = 20 size 12{4x+3y+2z="20"} {}
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x + 2y 4z = 1 size 12{x+2y - 4z=1} {}
2x 3y + 8z = 9 size 12{2x - 3y+8z=9} {}

( 3 4 / 7t size 12{3 - 4/7t} {} , 1 + 16 / 7t size 12{ - 1+"16"/7t} {} , t size 12{t} {} )

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Jessica has a collection of 15 coins consisting of nickels, dimes and quarters. If the total worth of the coins is $1.80, how many are there of each? Find all three solutions.

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The latest reports indicate that there are altogether 20,000 American, French, and Russian troops in Bosnia. The sum of the number of Russian troops and twice the American troops equals 10,000. Furthermore, the Americans have 5,000 more troops than the French. Are these reports consistent?

No, they are not consistent.

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Source:  OpenStax, Applied finite mathematics. OpenStax CNX. Jul 16, 2011 Download for free at http://cnx.org/content/col10613/1.5
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