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EQUATIONS Grade 9

Introduction. This is one of the hardest topics. Therefore I figured out two tricks they are very easy and they work most of the time.

1. The "Common Sens" Approach (1st trick).
E.g. Solve this equation:

3x + 4 = 46

To solve an equation means to find a value of the variable (in one case "x") that makes the statement true. You should say to yourself:

Something plus 4 = 46

so that something must be 42. Therefore, 3x = 42, which means 3 times something = 42. So it must be

42 ÷ 3 = 14

So x = 14. In other words don't think of it as algebra, but as "Find the mystery number".

2. The Trial and Error Method (2nd trick).
This is a perfectly good method, and although it won't work every time, it usually does, especially if the answer is a whole number. The big secret of trial and error method is to find two opposite cases and keep taking values in between.
Solve for x :

3x + 5 = 21 - 5x
Left Side (LS) = Right Side (RS)

Find a number that makes Right Side (RS) bigger, and then one that makes the Left Side (LS) bigger, and then try values in between them.

1 st case try a number that makes RS bigger:

x = 1
3x + 5 = 21 - 5x
3(1) + 5 = 21 - 5(1)
3 + 5 = 21 - 5
8 = 16
It is no good, RS two big

2nd case try a number that makes LS bigger:

x = 3
3x + 5 = 21 - 5x
3(3) + 5 = 21 - 5(3)
9 + 5 = 21 - 15
14 = 6
no good, LS two big

3rd case try in between:

x = 2
3x + 5 = 21 - 5x
3(2) + 5 = 21 - 5(2)
6 + 5 = 21 - 10
11 = 11
yes, so x = 2

Overall there are six steps to solve any type of equations:
To illustrate the sequence of all possible steps I am going to use a hard example (radical equation):

E.g
1st STEP. Get rid of any square root signs if any by squaring both sides:


2nd STEP. Get everything off the buttom by cross - multilping up to every other term: (L.C.D.)


2(2x+5) - (x + 4) = 9(2x + 5)

3rd STEP. Multiply out any brackets by using distributive properly:

2(2x + 5) - (x + 4) = 9(2x + 5)
4x + 10 - x - 4 = 18x + 45

4th STEP. Collect all uke term on the one side of the "=" and non-subject terms (constant terms) on the other side, remembering to remove the +/- sign of any term that crosses the "=" sign.

4x + 10 - x - 4 = 18x + 45

+18x moves across the "=" and becomes -18x;
+10 moves across the "=" and becomes -10;
-4 moves across the "=" and becomes +4

4x - x - 18x = 45 - 10 + 4

5th STEP. Combine together like terms on each side of the equation, and reduce it to the form "Ax = B"

-15x = 39

6th STEP. Finally to get x = B/A divide both sides by A in our case by -15.


 

Let's try these:
Example 1:   x - 9 = 35 (Simple Equations)
Example 2: 5x + 54 = 4x - 41 (Solving Equations by Addition and Subtraction)
Example 3: 4(x - 2) = 3(x - 4) - 5 (Solving Equations by Division)
Example 4: 3(2x - 1) - 4 = 3x - 6 (Solving Equations by Division)
Example 5: (Solving Equations by Multiplication)
Example 6: 2(x - 3)(x + 2) - 5 = (2x - 1)(x + 2)  (Solving Equations Involving Polynomials)

Example 1: By using the rules we set the (Simple Equations) variable terms on one side of the equation and the constant terms on the other side.

x - 9 = 35

-9 moves across the "=" and becomes +9

x = 35 + 9
x = 44

Check L.S. = x - 9
L.S. = 44 - 9
L.S. = 35
L.S. = -421

L.S. = R.S. therefor 44 is the solution of the equation.


Example 2: (Solving Equations by Addition and Subtraction)

5x + 54 = 4x - 41

Again we set the variable terms on one side of the equation and the constant terms on the other side.
+4x moves across the "=" and becomes -4x
+54 moves across the "=" and becomes -54

5x - 4x = - 41 - 54
x = - 95

Check L.S. = 5x + 54 R.S. = 4x - 41
L.S. = 5(-95) + 54 R.S. = 4(-95) - 41
L.S. = -475 + 54 R.S. = -380 - 41
L.S. = -421 R.S. = -421
L.S. = R.S. therefor x = -95 is the solution of the equation.

Example 3: (Solving Equations by Division)

4(x - 2) = 3(x - 4) - 5

First multiply out any brackets by using distributive property:

4x - 8 = 3x - 12 - 5

Then get the variable terms on one side of the equation and the constant terms on the other.
+3x moves across the "=" and becomes -3x
-8 moves across the "=" and becomes +8

4x - 3x = 8 - 12 - 5
x = -9

Check L.S. = 4x - 8 R.S. = 3x - 12 - 5
L.S. = 4(-9) - 8 R.S. = 4(-9) - 12 - 5
L.S. = -44 R.S. = -44
L.S. = R.S. therefor -9 is the root of the equation.

Example 4: (Solving Equations by Division)

3(2x - 1) - 4 = 3x - 6

Distributive property

6x - 3 - 4 = 3x - 6

Set the variable terms on one side of the equation and the constant terms on the other.

6x - 3x = 3 + 4 - 6

Colect like terms

3x = 1

Divide both sides by 3



Check L.S. =  3(2x - 1) - 4 R.S. =  3x - 6
L.S. =  R.S. = 
L.S. =  R.S. =  1 - 6
L.S. =  R.S. =  - 5
L.S. =  - 1 - 4
L.S. =  - 5
L.S. = R.S. therefor the solution is x = 1/3.

Example 5: (Solving Equations by Multiplication)

First, set everything off the bottom by cross - multiplyng up to every other term (The L.C.D. for 3 amd 2 is 6).

Multiply

2(x + 1) - 42 = 3(x - 1)

Distributive property

2x + 2 - 42 = 3x - 3

Set the variable terms on one side of the equation and the constant terms on the other side.

2x - 3x = 42 - 2 - 3

Collect like trms

-x = 37

Multiply by (-1) to set positive x

(-1)(-x) = (-1)(37)
x = - 37

Check L.S. =  R.S. = 
L.S. =  R.S. = 
L.S. =  R.S. = 
L.S. =  -12 - 7 R.S. =  - 19
L.S. =  -19
L.S. = R.S. therefore the solution is -37.

Example 6: (Solving Equations Involving Polynomials)

2(x - 3)(x + 2) - 5 = (2x - 1)(x + 2)

Foil

Remove brackets by using distributive property

Set the variable terms onone side of the equation and the constant terms on the ither side.


-5x = +15

Divide both sides by -5

x = -3

Check L.S. = 2(x - 3)(x + 2) - 5 R.S. = (2x - 1)(x + 2)
L.S. = 2(-3 - 3)(-3 + 2) - 5 R.S. = [2(-3) - 1](-3 + 2)
L.S. = 2(-6)(-1) - 5 R.S. = (-6 - 1)(-1)
L.S. = 12 - 5 R.S. = (-7)(-1)
L.S. = 7 R.S. = 7
L.S. = R.S. therefore the root of the equation i -3.