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Illustrative Math
Grade 8
Let’s solve equations with different numbers of solutions.
Illustrative Math Unit 8.4, Lesson 8 (printable worksheets)
Sometimes it’s possible to look at the structure of an equation and tell if it has infinitely many solutions or no solutions. For example, look at
2(12x + 18) + 6 = 18x + 6(x + 7)
Using the distributive property on the left and right sides, we get
24x + 36 + 6 = 18x + 6x + 42
From here, collecting like terms gives us
24x + 42 = 24x + 42
Since the left and right sides of the equation are the same, we know that this equation is true for any value of x without doing any more moves!
Similarly, we can sometimes use structure to tell if an equation has no solutions. For example, look at
6(6x + 5) = 12(3x + 2) + 12
If we think about each move as we go, we can stop when we realize there is no solution:
1/6 · 6(6x + 5) = 1/6 · (12(3x + 2) + 12) Multiply each side by 1/6
6x + 5 = 2(3x + 2) + 2 Distribute 1/6 on the right side.
6x + 5 = 6x + 4 + 2 Distribute 2 on the right side.
The last move makes it clear that the constant terms on each side, 5 and 4 + 2, are not the same. Since adding 5 to an amount is always less than adding 4 + 2 to that same amount, we know there are no solutions.
Doing moves to keep an equation balanced is a powerful part of solving equations, but thinking about what the structure of an equation tells us about the solutions is just as important.
Consider the unfinished equation 12(x + 3) + 18 = . Match the following expressions with the number of solutions the equation would have with that expression on the right hand side.
Your teacher will give you some cards.
For each equation, determine whether it has no solutions, exactly one solution, or is true for all values of x (and has infinitely many solutions). If an equation has one solution, solve to find the value of x that makes the statement true.
Consecutive numbers follow one right after the other. An example of three consecutive numbers is 17, 18, and 19. Another example is -100, -99, -98.
The average is always the second number.
The average is always the second number because the number to the left is one smaller and the number to the right is one bigger.
The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.
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