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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 |
 |
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| 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.
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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) |
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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
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| 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

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| Check |
L.S. = |
3(2x - 1) - 4 |
R.S. = |
3x - 6 |
|
L.S. = |
 |
R.S. = |
 |
|
L.S. = |
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R.S. = |
1 - 6 |
|
L.S. = |
 |
R.S. = |
- 5 |
|
L.S. = |
- 1 - 4 |
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|
|
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).

2(x + 1) - 42 = 3(x - 1)
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
-x = 37
Multiply by (-1) to set positive x
(-1)(-x) = (-1)(37)
x = - 37
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| Check |
L.S. = |
 |
R.S. = |
 |
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L.S. = |
 |
R.S. = |
 |
|
L.S. = |
 |
R.S. = |
 |
|
L.S. = |
-12 - 7 |
R.S. = |
- 19 |
|
L.S. = |
-19 |
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|
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| 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
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 |
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| L.S. = R.S. therefore the root of the equation i -3. |
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