- #1
jgthb
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Hi
I just found an old high school math exercise and was wondering how it should be answered mathematically correct. The exercise is:
Given lim f(x)=(x^2+x+a)/(x-1) = 3 for x->1, determine a. Back in high school I used to say something like this:
Since the denominator goes to 0 for x -> 1 the fraction must be reducible, so x=1 must be a root in the numerator, and thus we can write it as (x-1)(x-b). Hence:
f(x)=(x-1)(x-b)/(x-1)=x-b.
For x->1 we get:
1-b=3 => b=-2, and thus:
(x-1)(x+2)=x^2+x-2,
so a=-2.
The part I think lacks rigour is the argument that the fraction must be reducible. I guess you have to go on about it in some other way. Any suggestions?
/Jonathan
I just found an old high school math exercise and was wondering how it should be answered mathematically correct. The exercise is:
Given lim f(x)=(x^2+x+a)/(x-1) = 3 for x->1, determine a. Back in high school I used to say something like this:
Since the denominator goes to 0 for x -> 1 the fraction must be reducible, so x=1 must be a root in the numerator, and thus we can write it as (x-1)(x-b). Hence:
f(x)=(x-1)(x-b)/(x-1)=x-b.
For x->1 we get:
1-b=3 => b=-2, and thus:
(x-1)(x+2)=x^2+x-2,
so a=-2.
The part I think lacks rigour is the argument that the fraction must be reducible. I guess you have to go on about it in some other way. Any suggestions?
/Jonathan