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Big Ideas Math: Modeling Real Life, Grade 7 – Solving Equations Using Addition or Subtraction

Equation: y + (- 3) = - 8
Solution: y = - 5

Solution

We have been given a sentence and we need to translate it into an equation.

The sum of a number y and - 3 is - 8.

Every equation has an equals sign and values or expressions on either side of it. Key phrases, such as is, is equal to, and equals tell us about the placement of the equals sign. The sum of a numberyand- 3 is - 8. ⇕ The sum of a numberyand- 3 = - 8. On the left-hand side we have one key word: sum. This word tell us the operation that will be used in our equation. Sum indicates addition. Sum of a numberyand - 3 y + ( - 3) Putting the sides together, we have a complete equation. y + ( - 3) = - 8 Finally, we can solve the equation using the Addition Property of Equality.

y + (- 3) = - 8
AddNeg

a+(- b)=a-b

y - 3 = - 8
AddEqn

LHS+3=RHS+3

y - 3 + 3 = - 8 + 3
AddTerms

Add terms

y = - 5
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