<s>[INST] Why am I getting inconsistent limit results? My objective is to solve the following limit $$\lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}},$$ where $a$ and $b$ are real constants, such that $a \gt b \gt 0$.
I (apparently) managed to solve it in two different ways, leading to two distinct answers, which of course, means something is wrong.
Method 1: 
$$\lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}\frac{a^{-x}}{a^{-x}}=\lim_{x \to \infty} \frac{1 - (\frac{b}{a})^x}{a - a(\frac{b}{a})^{x+1}}$$
using the L´Hospital rule, we get
$$\lim_{x \to \infty} \frac{-(\log{\frac{b}{a}})(\frac{b}{a})^x}{-a(\log{\frac{b}{a}})(\frac{b}{a})^{x+1}} = \frac{1}{a}(\frac{b}{a})^{-1} = \frac{1}{b}.$$
Method 2: 
$$\lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}\frac{b^{-x}}{b^{-x}}=\lim_{x \to \infty} \frac{(\frac{a}{b})^x - 1}{b(\frac{a}{b})^{x+1} - b}$$
using the L´Hospital rule, we get
$$\lim_{x \to \infty} \frac{(\log{\frac{a}{b}})(\frac{a}{b})^x}{b(\log{\frac{a}{b}})(\frac{a}{b})^{x+1}} = \frac{1}{b}(\frac{a}{b})^{-1} = \frac{1}{a}.$$
What am I doing wrong?
Edit: added constraints for $a$ and $b$.
 [/INST] Note that after this step
$$\lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}\frac{a^{-x}}{a^{-x}}=\lim_{x \to \infty} \frac{1 - (\frac{b}{a})^x}{a - a(\frac{b}{a})^{x+1}}$$
we need some condition to apply l'Hopital (i.e. indeterminate form $\frac{\infty}{\infty}$ or $\frac{0}{0}$) and similarly for Method 2. That's not the correct way to solve the limit.
To solve note that for $x$ real we need $a,b>0$ and we can distinguish the two cases $a>b$ or $b>a$. For $x$ integer we can also consider $a,b <0$.
Notably for $x$ real and $a,b>0$


*

*for $a>b\implies \lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}\frac{a^{-x}}{a^{-x}}=\lim_{x \to \infty} \frac{1 - (\frac{b}{a})^x}{a - a(\frac{b}{a})^{x+1}}=\frac1a$

*for $b>a\implies \lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}\frac{b^{-x}}{b^{-x}}=\lim_{x \to \infty} \frac{(\frac{a}{b})^x - 1}{b(\frac{a}{b})^{x+1} - b}=\frac1b$
For $x$ integer with $a\cdot b<0$


*

*for $|a|>|b|\implies \lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}=\lim_{x \to \infty} \frac{a^{x}}{a^{x+1}}\frac{1 - (\frac{b}{a})^x}{1 - (\frac{b}{a})^{x+1}}=\frac1a$

*for $|b|>|a|\implies \lim_{x \to \infty} \frac{a^x - b^x}{a^{x+1} - b^{x+1}}=\lim_{x \to \infty} \frac{b^{x}}{b^{x+1}}\frac{(\frac{a}{b})^x - 1}{(\frac{a}{b})^{x+1} - 1}=\frac1b$
</s>