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Lesson · Grade 8 · Mathematics

Substituting into Expressions with Negatives and Decimals

  • Grade 8
  • Mathematics
  • 2 min 2 s
  • British Columbia
  • BC8-MA-21

The video puts 14 in brackets in place of each n in 0.5n − 3n + 25, warns that 3n is not 314, multiplies first and then adds and subtracts from left to right to get −10. It then repeats the steps with n = −2, showing how the brackets keep the negative sign with its number and that subtracting −6 means adding 6, to get 30.

Concept: Substituting into an expression with negatives and decimals

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In one sentence

The question this lesson answers

Let us evaluate this expression when n is 14. To evaluate means to work out its value.

This expression has numbers, a letter and operations. The letter n is a variable: it stands for a number.

Transcript

What the lesson says

Let us evaluate this expression when n is 14. To evaluate means to work out its value.

This expression has numbers, a letter and operations. The letter n is a variable: it stands for a number.

A number written next to a letter means multiply. So 0.5 n means 0.5 times n, and 3 n means 3 times n.

To substitute, put the number in place of the letter. Write 14 in brackets in place of each n.

A common mistake is to write 3 n as 314. The brackets show that 3 n means 3 times 14.

Multiply first. 0.5 times 14 is 7, and 3 times 14 is 42.

Then add and subtract from left to right. 7 minus 42 is negative 35, and negative 35 plus 25 is negative 10.

So when n is 14, the expression equals negative 10.

Now let n equal negative 2. Put negative 2 in brackets in place of each n, so the negative sign stays with its number.

Multiply first. 0.5 times negative 2 is negative 1, and 3 times negative 2 is negative 6.

Subtracting negative 6 is the same as adding 6.

Now add from left to right. Negative 1 plus 6 is 5, and 5 plus 25 is 30.

So when n is negative 2, the expression equals 30.

To recap: put the number in brackets in place of the letter, multiply first, then add and subtract from left to right.

Curriculum

Where this fits

British Columbia, Grade 8

Mathematics, Patterns and algebra: expressions: writing and evaluating using substitutionBC8-MA-21

Previous: Writing an expression to describe a relationshipNext: Solving Two-Step Equations with Negative Numbers

The outcomes and strands follow the British Columbia curriculum.

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