Year 9 Inequalities Dr J Frost ([email protected]) Last modified: 23 rd March 2015...
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Transcript of Year 9 Inequalities Dr J Frost ([email protected]) Last modified: 23 rd March 2015...
Year 9 Inequalities
Dr J Frost ([email protected])
Last modified: 23rd March 2015
Objectives: Solving linear inequalities, combining inequalities and representing solutions on number lines.
Means: x is less than or equal to 4.
Writing inequalities and drawing number linesYou need to be able to sketch equalities and strict inequalities on a number line.
x > 3Means: x is (strictly) greater than 3.
0 1 2 3 4 5
?
This is known as a βstrictβ inequality. x < -1
Means: x is (strictly) less than -1.
-3 -2 -1 0 1 2
?
x β₯ 4Means: x is greater than or equal to 4.
2 3 4 5 6 7
?x β€ 5
2 3 4 5 6 7
?
? ?
? ?
Deal or No Deal?We can manipulate inequalities in various ways, but which of these are allowed and not allowed?
Click to Deal Click to No Deal
π>π Can we add or subtract to both sides?
πβπ>π
Deal or No Deal?We can manipulate inequalities in various ways, but which of these are allowed and not allowed?
Click to Deal Click to No Deal
π π>π
π>π
Can we divide both sides by a positive number?
Deal or No Deal?We can manipulate inequalities in various ways, but which of these are allowed and not allowed?
Click to Deal Click to No Deal
π<π
π π<π
Can we multiply both sides by a positive number?
Deal or No Deal?We can manipulate inequalities in various ways, but which of these are allowed and not allowed?
Click to Deal Click to No Deal
π<π
βπ<βπ
Can we multiply both sides by a negative number?
2
If we multiply or divide both sides of the inequality by a negative number, the inequality βflipsβ!
< 4
Click to start Bro-manimation
-2 -4OMG magic!
βFlippingβ the inequality
Alternative Approach
Or you could simply avoid dividing by a negative number at all by moving the variable to the side that is positive.
βπ₯<3 1β3 π₯β₯7?
?
?
?
?
?
Solve
Solve
Solve
Solve
Solve
Quickfire Examples
2 π₯<4
βπ₯>β3
4 π₯β₯12
β4 π₯>4βπ₯2β€1
π₯<2
π₯<3
π₯β₯3
π₯<β1π₯β₯β2
?
?
?
?
?
Deal or No Deal?We can manipulate inequalities in various ways, but which of these are allowed and not allowed?
Click to Deal Click to No Deal
Can we multiply both sides by a variable?
The problem is, we donβt know if the variable has a positive or negative value, so negative solutions would flip it and positive ones wouldnβt. You wonβt have to solve questions like this until Further Maths A Level!
1π₯
<2
1<2 π₯
Solve
Solve
Solve
Solve
Solve
Hint: Do the addition/subtraction before you do the multiplication/division.
3 π₯β4<20
4 π₯+7>35
5+π₯2β₯β2
7β3π₯>4
6βπ₯3β€1
π₯<8
π₯>7
π₯β₯β14
π₯<1
π₯β₯15
?
?
?
?
?
More Examples
8 < 5x - 2 β€ 23
Hint: Do the addition/subtraction before you do the multiplication/division.
π<π β€π
8 < 5x - 25x - 2 β€ 23and
2 < x and x β€ 52 < x x β€ 5
Click to start bromanimation
Dealing with multiple inequalities
Solveβπ<π<π?
Hint: Do the addition/subtraction before you do the multiplication/division.
Solveβπ<π<π?
More Examples
π<π π+π<π
βπ<βπ<π
Test Your Understanding
ππ<ππ βπ<ππSolve
π<π<πSolve
βπ<π<π
?
?π<πβπ π<π
2 π₯β1>5 π>πExercise 1Solve the following inequalities, and illustrate each on a number line:
1
2
3
4
5
6
7
8
9
10
11
N1 Sketch the graphs for and . Hence solve 0 < x < 1
?
??
?
???
??
?
??N2
You can get around the problem of multiplying/dividing both sides by an expression involving a variable, by separately considering when itβs positive, and when itβs negative, and putting this together.Hence solve:
If we assume is positive, then and solving gives . Thus as we had to assume . If then this solves to which is a contradiction.Thus
?
Combining inequalitiesItβs absolutely crucial that you distinguish between the words βandβ and βorβ when constraining the values of a variable.
x β₯ 2 and x < 4
AND How would we express βx is greater than or equal to 2, and less than 4β?
x β₯ 2, x < 4
2 β€ x < 4
This last one emphasises the fact that x is between 2 and 4.
?
?
?
OR How would we express βx is less than -1, or greater than 3β?
x < -1 or x > 3?
This is the only way you would write this β you must use the word βorβ.
Combining inequalitiesItβs absolutely crucial that you distinguish between the words βandβ and βorβ when constraining the values of a variable.
2 β€ x < 4 x < -1 or x > 4
0 1 2 3 4 5
?
-1 0 1 2 3 4
?
Combining inequalitiesItβs absolutely crucial that you distinguish between the words βandβ and βorβ when constraining the values of a variable.
x β₯ 2 and x < 4 x < -1 or x > 4
0 1 2 3 4 5
?
-1 0 1 2 3 4
?
or and
To illustrate the difference, what happens when we switch them?
I will shoot you if I see any of theseβ¦
4>π₯<8
4<π₯>7
7 > > 4This is technically equivalent to:x > 7
This is technically equivalent to:x < 4
The least offensive of the three, but should be written:4 < x < 7
?
?
?
Combining Inequalities
2 5
4
2 5
4
2<π₯<5
π₯<4
Combined
Combined
π>π
π<π<π
?
?
In general, we can combine inequalities either by common sense, or using number lines...
Where are you on both lines?