Exercise 39 Page 9

Looking at the erroneous solution, we can see that the mistake made was related to whether - 0.8 should be added or subtracted from both sides of the equation. The critical mistake was that - 0.8 was added to both sides, rather than subtracted. Incorrect:r=12.6 + (- 0.8) Correct:r=12.6 - (- 0.8) One way to avoid this mistake would be to apply the Commutative Property of Addition to rearrange the terms on the left-hand side of the equation. See our correct solution below for an example.

- 0.8+r=12.6

CommutativePropAdd

Commutative Property of Addition

r+(- 0.8)=12.6

AddNeg

a+(- b)=a-b

r-0.8=12.6

AddEqn

LHS+0.8=RHS+0.8

r-0.8+0.8=12.6+0.8

AddTerms

Add terms

r=13.4