Formative Assessment 51

Assessment Problem: 

1) 15 – x =10
2) 6 – x = 8
3) -3 –  x = 5
4) -x = 6
5) 2x – 4 = x + 2
6) -5x + 6 = 2x + 20
7) 2– 3x + 6 = 2x – 12
8) -2( 3x – 4) = -4x – 10

Mathematical Issues: 

Errors are made involving the negative sign during the simplification process. There are a variety of ways to misuse the negative sign. Misuse of the commutative property, ignoring the negative sign, assigning the negative to the wrong term. Always subtracting whether addition or subtraction that is being undone.

Common Responses: 

1)  x =5 2)  x = 2  (corr. -2) 3)  x =2     (corr. -8)
4)   x = 6 (x =-6)    5)  x = -2 (x = 6)
7) x= 16   (x =4)   8)  x = 0   (undefined or no solution)

Domain: 
Algebraic Relations
Theme: 
Solving equations
Research References: 
Reference Vlassis, J. (2001) “Solving Equations with Negatives or Crossing the Formalizing Gap.” Faculty of Psychology and of Sciences of Education-University of Liege- Belgium. PME-25