Stage 4 -algebra - algebraic techniques

Use the algebraic symbol system to expand and simplify simple algebraic expressions; substitute into algebraic expressions.

Strategy

Students can:

  • use the algebraic symbol system to expand and simplify simple algebraic expressions
  • substitute into algebraic expressions

Activities to support the strategy

Activity 1

Teacher introduces the example of a forensic scientist who uses skeletal remains to infer a person's height from a few bones. Anthropologists have developed formulas to estimate a person's height from the length of long bones such as the humerus and the femur. These formulas change depending on the person's gender and race.

The humerus (single bone extending from the elbow to the shoulder socket) may be used to obtain an indication of a person's height using formulas such as these:

  • Female: height = 2.8 x length of the humerus (in mm) + 704 or H = 2.8L + 704
  • Male: height = 2.9 x length of the humerus (in mm) + 695 or H = 2.9L + 695

1. Students are given these formulas and they are asked to use the relevant one to estimate their heights (the group may first need to be guided through an example on how to substitute values into the formulas). Students work in pairs to check the accuracy of the formulas by measuring their heights directly to complete the information below. Please record their:

  • Length of their humerus
  • Height according to the formula
  • Actual height

2. In groups, students apply formulas that estimate the height of a person by using four of the body's long bones to solve an archaeological problem.

Mystery of the Bones

1. The scenario

The remains of four girls have been discovered in a recent archaeological find. There are four bones from each girl left but archaeologists don't know which bones belong together. You have been engaged as the mathematician to work out which bones belong to which girl and approximately how tall the girls were.

2. Background clues

By knowing the length of certain bones in the body, scientists can estimate a person's height. The bones used are the humerus (shoulder to elbow), radius (elbow to wrist), femur (thigh bone), and tibia (knee to ankle).

The formulas for approximate heights (in mm) for males and females are shown in this table

 
Male heightFemale height
2.9 x (Length of humerus) + 6952.8 x (length of humerus) + 704
3.3 x (Length of radius) + 8463.3 x (length of radius) + 800
1.9 x (Length of femur) + 8002 x (length of femur) + 717
2.4 x (Length of tibia) + 7742.4 x (length of tibia) + 736

Example

For a tibia belonging to a male and which is 45cm long, we would estimate the height by using the formula:

H = 2.4L + 774

Converting the length to millimetres first, we substitute L = 450 to obtain:

  • H = 2.4 x 450 + 774
  • H = 1854mm

Therefore, the tibia belongs to a man who is about 1.85 metres tall.

3. Your task

Look at the data table showing the length of various bones found at the site. The bones are numbered from 1 to 16 and the measurements are in millimetres. By examining each bone in turn, calculate the approximate height of the girl for each bone. Then match the bones to discover which bones go together and find the approximate height of each girl.

Bone calculation approximate height

Please enter the approximate heights:

Girl A:                        Girl B:

Girl C:                        Girl D:

Data DataDataData
1
Humerus
235
2
Tibia
385
3
Femur
470
4
Tibia
407
5
Radius
170
6
Radius
210
7
Radius
260
8
Humerus
280
9
Femur
320
10
Femur
495
11
Tibia
260
12
Humerus
360
13
Tibia
315
14
Humerus
340
15
Radius
275
16
Femur
385

Activity 2

1. Pose this problem: Is 6 (2 + 3) = 6 x 2 + 3?

Work through the problem on the board by following these steps.

  • Ask: What is 6 (2 + 3) equal to?
  • Show how to work through the part in brackets first: 2 + 3 = 5, so 6 (2 + 3) = 6 x 5
  • Draw an array of dots to show 6 (2 + 3) = 6 x 5
  • Ask: What is 6 x 2 + 3 equal to?
  • There are no brackets, so emphasise that the multiplication is done before the addition.
  • Draw an array of dots to show 6 x 2 = 12, then + 3

  • Remind students of the order of operations when solving number sentences.
  • The array shows that 6 (2 + 3) is not equal to 6 x 2 + 3.

2. Have the students investigate problems of the following type in small groups:

Kyle wrote 6 (11 + 7) = 6 x 11 + 7 in his maths book.

Ask students to:

  • do the calculations to show the statement is incorrect
  • draw an array of dots to convince Kyle that his statement is incorrect.

3. Have students explore the following problem: Determine and justify whether a simplified algebraic expression is correct by substituting numbers for letters, - does 2x + 3x = 5x when x = 2?

4. Ask students to develop algebraic expressions for their partner to explore, then discuss and check their answers as a class.

5. Students use the Exploring linear equations learning object and video to explore linear equations. This video shows how linear equations are used to calculate how much it costs to feed the animals at a zoo.

  • e.g. The daily cost of meat to feed the tigers is ($9.90 per kg x 36 kg) + $10 delivery = $366.40. The interactive resource allows students to change variables in linear equations to find different ways of obtaining the same solution. Included are print activities.

6. Use spreadsheets to investigate formulas such as p=2(l+b), a=½ bh, v=lbh, a2 = b2 +c2 through substitution.

Activity 3

1. As a class, discuss the similarities and differences in these expressions:

 
No1st Term2nd TermLike/Unlike
15a6alike
22b4blike
37c4clike
48ddlike
57e3funlike
66g5glike
74h4hlike
 
No1st Term2nd TermLike/Unlike
82j3kunlike
93m8nunlike
104p47p4like
116q2runlike
129s5s3unlike
139u4ulike
144v27vunlike

Ask students to formulate a definition of like terms.

Like terms are algebraic expressions which have the same  (insert answer)  to the same  (insert answer) (pronumeral, power).

2. As a class, determine which terms are like in these expressions:

NoExpressionLike terms
13a + 3b +7a3a, 7a
25c + 8d - 4c5c, -4c

3. As a class, simplify these expressions by first collecting like terms

 
1    3a + 3b + 7a
=  3a + 7a + 3b
= 10a       +  3b

collect like terms

simplify

2   5c + 8d - 4c
= 5c - 4c + 8d
= c         + 8d

collect like terms

simplify

4. Lead the class in expanding the grouping symbols and simplifying these expressions:

 
1     3 (a + 3b) + 7a
=   3 x a +3 x 3b +7a
=   3a + 9b + 7a
=   3a + 7a + 9b
=   10a       +9b
expand grouping symbols


collect like terms
simplify

Activity 3 – independent

1.Change these relationships from words to symbols

 
DescriptionWordsSymbols
1Start at 13 and add 2 each time13, 15, 17, 19 and so on

2. Use the cards from the symbols and words sheet to play Concentration.

References

Australian curriculum reference: ACMNA176

Create algebraic expressions and evaluate them by substituting a given value for each variable. ACMNA177: Extend and apply the laws and properties of arithmetic to algebraic terms and expressions.

NSW syllabus reference: MA4-8NA

Generalises number properties to operate with algebraic expressions.

NSW literacy continuum reference: COMC13M11

Comprehension, Cluster 13, Marker 11: Locates and synthesises information to draw conclusions from a variety of sources.

Teacher resources

Curriculum support

Interactive Whiteboard Activities

Interactive Games

Numeracy Apps

Hot potato: This is a game to help you practice your Algebra! The main mode is “potato” mode, based on the classic game of “Hot Potato”. You play with up to 5 friends and the goal is to answer Algebra questions as quickly as possible, because as you play, there is an invisible timer counting down and as soon as it runs out, the current player is eliminated. If you answer incorrectly, five seconds is taken off the timer and you must answer the question again until you get it right. The goal is to be the last player standing.

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