Grade 8Mathematics

Algebraic Expressions

Forming, simplifying, factorising algebraic expressions; binomial expansion.

📖 6 min read · 3 worked examples · 4 practice questions

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The lesson

By the end of this session, you'll be able to form, simplify, factorise, and expand binomials, and see how these ideas show up in everyday Kenyan life. First, we'll learn how to write algebraic expressions from word problems—think of calculating the total cost of buying mangoes at the market. Next, we'll practice simplifying them, just like reducing a recipe's ingredients to a single amount. Then we'll factorise, which is like breaking a harvest into equal groups. Finally, we'll explore binomial expansion, a tool you'll need when dealing with area calculations for farm plots. Who can give an example of a simple expression from daily life? For instance, if a vendor sells bananas for 30 shillings each, the cost for x bananas is 30x shillings. That's an algebraic expression right away. We'll move step by step through each skill, checking understanding along the way. Feel free to raise your hand if anything is unclear—remember, every question helps us all learn better.

Class, today we're looking at Forming Algebraic Expressions—turning words into math. First, we need to identify the variables and constants in a problem. Variables are the unknowns we solve for, and constants are the numbers that stay the same. For example, in the sentence "Each mango costs Ksh 20," the number 20 is a constant, while the number of mangoes would be a variable. At this table. It shows a real‑world Kenyan scenario: the price of mangoes. Notice how we label the variable m for the number of mangoes, keep 20 as a constant, and write the expression 20m. A farmer harvests 5 bags of maize. Each bag weighs x kilograms. How would you write an expression for the total weight? Think of the constant (5) and the variable (x).

Everyone, today we're focusing on simplifying algebraic expressions. We'll look at three main tools: combining like terms, the distributive property, and a handy flowchart to guide you. First, combine like terms. That means any terms that have the same variable part—like 3x and 5x—can be added by summing their coefficients. 3x + 5x becomes 8x. Does anyone see why we can't combine 2x and 3y? Because the variable parts are different. Next, the distributive property: a(b + c) = ab + ac. Use this when a term multiplies a parenthesis. For example, 4(2x + 3) becomes 8x + 12. Let's apply what we've learned to a real Kenyan scenario: a boda‑boda rider travels 3d + 2d kilometers each day. Combining like terms gives 5d kilometers total. Remember: first combine like terms, then use the distributive property when needed, and consult the flowchart if you get stuck. Any quick questions before we move on?

Welcome, everyone. Today we're focusing on factorising algebraic expressions, a key skill for simplifying problems. First, we always look for the greatest common factor, or GCF, of the terms. That's our stepping stone for all further factoring. When we spot a GCF, we pull it out, just like this: a·b + a·c = a (b + c). Notice how the 'a' appears once outside the parentheses. Another useful pattern is the difference of squares: a² – b² = (a – b)(a + b). This works whenever you have two squares subtracted from each other. Let's apply these ideas to a Kenyan context: two square fields, each 10 m on a side, placed side‑by‑side. The total fence length around both fields can be found by factoring the expression 20 m + 2·10 m, revealing the same concept of extracting a common factor.

Class, let's explore the simple case of binomial expansion. Our title here reads Binomial Expansion (Simple Case), and we'll see how the pattern (a + b)² works. First, expand (a + b)². The result is a² + 2ab + b². Notice the three terms: the squares of each part and twice their product. You can picture this as a square whose side length is a + b. Each term corresponds to an area: a² is the big square of side a, b² is the square of side b, and 2ab represents the two rectangles that fill the remaining space. Let's apply the idea to a real Kenyan example: suppose k represents the number of goats in a pen, and we add three more goats. We want to find (k + 3)². We start with the expression (k + 3)². Expanding using the pattern gives k² + 2·k·3 + 3². Simplifying the middle term, 2·k·3 equals 6k, so the final expanded form is k² + 6k + 9. This tells us the total area of a square plot whose side length is k + 3 meters. To recap: we identified each part of the expansion, linked them to areas of a square, and worked through the concrete example of (k + 3)². Any questions before we move on?

We've reached the end of today's lesson. This slide is our summary and next steps. First, remember how we formed, simplified, factorised, and expanded binomial expressions. Those four skills are the core toolbox you now have. Second, the Kenyan examples we worked through—like calculating the area of a farm plot—show how these algebraic tricks solve real‑world problems you might see in everyday life. Third, your practice worksheet contains four extra problems. Finally, I encourage each of you to create your own expressions from situations around you—maybe the cost of a bus ride or the height of a mango tree. Turn those situations into algebra, and you'll see how useful it really is. Great work, everyone! See you next class.

Worked examples

– Forming Expressions

Let's work through our first example: forming an algebraic expression from a real‑world market problem. The problem says a vendor sells 3 bags of maize at Ksh 250 each and 2 bags of beans at Ksh 180 each. We need to turn that description into a mathematical expression. First, we define our variables. Let m represent the number of maize bags and b represent the number of bean bags. We can write the total cost as the sum of the cost of maize and the cost of beans: 250 m + 180 b. To check, if we plug in m = 3 and b = 2, we get 250 × 3 + 180 × 2, which matches the numbers given in the problem.

– Simplification

Let's work through Worked Example 2, where we simplify an expression that represents a farmer's profit. Here is the original expression: 5x + 3x − 2y + 4y. The variables x and y could stand for the number of corn and wheat units sold. First, we combine like terms. The x‑terms (5x + 3x) give us 8x, and the y‑terms (‑2y + 4y) give us 2y. The simplified profit expression is 8x + 2y. This tells us the total profit depends on eight times the corn revenue plus two times the wheat revenue. Any questions before we move on? Remember, combining like terms is just grouping the same kind of crop together.

– Factorising

Let's work through Example 3, where we factorise a simple expression that could represent a school‑fee discount. First, look at the expression we need to factor: 6t – 9. The greatest common factor, or GCF, of the two terms is 3. Here is the original expression written as a LaTeX formula: 6t – 9. Factoring out the GCF 3 gives us 3 (2t – 3). That's the factorised form. To be sure, we can expand 3 (2t – 3) back out: 3·2t = 6t and 3·(‑3) = ‑9, so we recover 6t – 9. Interpretation: if t represents the number of months a student pays after a discount, the factor 3 shows the common savings per month, and (2t – 3) reflects the adjusted total fee.

Practice questions

  • Think about how we translate a real‑world buying situation into math. You purchase 3 bags of rice at Ksh 120 each and 2 boxes of beans at Ksh 150 each.
  • Asks you to combine like terms. Adding the x‑terms 4x and 7x gives 11x, and the –3y stays separate.
  • In Question 3 we factor the common factor from 12m – 8. The greatest common factor is 4, giving 4(3m – 2).
  • Finally, Question 4 tests binomial expansion. Remember (a + b)² = a² + 2ab + b².

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