Chapter 9 - Basic Algebra - 9.2 Adding and Subtracting Signed Numbers - 9.2 Exercises - Page 638: 17

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To add $-10+(-10)$, we can take the absolute value of both numbers. $|-10|=10$ $|-10|=10$ To add two negative numbers, we can add the absolute value of both numbers and then add a negative sign in front of the result. $|-10|+|-10|=10+10=20$ Therefore, $-10+(-10)=-20$.