Asked by Anonymous
in solving the equation (x+4)(x-4)=9 eric stated the solution would be x+4=9 =>5 or x-4=9 =>13 however at least one of these solutions fails to work when subsituted back into the original equation. why is that? solve the problem yourself and explain your reasoning
Answers
Answered by
drwls
(x+4)(x-4) = 9
x^2 -16 = 9
x^2 = 25
x = +5 or -5
Eric's logic is incorrect.
You cannot conclude from (x+4)(x-4)=9
than x-4 = 9.
In the other case, x + 4 = 9 does happen to be a solution, but that is a bit of a lucky guess.
x^2 -16 = 9
x^2 = 25
x = +5 or -5
Eric's logic is incorrect.
You cannot conclude from (x+4)(x-4)=9
than x-4 = 9.
In the other case, x + 4 = 9 does happen to be a solution, but that is a bit of a lucky guess.