Question
Evaluate the following limits: $\lim\limits_{\text{x}\rightarrow\infty}\frac{\sqrt{\text{x}^2+\text{a}^2}+\sqrt{\text{x}^2+\text{b}^2}}{\sqrt{\text{x}^2+\text{c}^2}+\sqrt{\text{x}^2+\text{d}^2}}$

Answer

$\lim\limits_{\text{x}\rightarrow\infty}\frac{\sqrt{\text{x}^2+\text{a}^2}+\sqrt{\text{x}^2+\text{b}^2}}{\sqrt{\text{x}^2+\text{c}^2}+\sqrt{\text{x}^2+\text{d}^2}}$ Rationalising the numerator and the denominator: $=\lim\limits_{\text{x}\rightarrow\infty}\Bigg[\frac{\big(\sqrt{\text{x}^2+\text{a}^2}-\sqrt{\text{x}^2+\text{b}^2}\big)}{\big(\sqrt{\text{x}^2+\text{c}^2}-\sqrt{\text{x}^2+\text{d}^2}\big)}\times\frac{\big(\sqrt{\text{x}^2+\text{c}^2}-\sqrt{\text{x}^2+\text{d}^2}\big)}{\big(\sqrt{\text{x}^2+\text{c}^2}-\sqrt{\text{x}^2+\text{d}^2}\big)}\times\frac{\big(\sqrt{\text{x}^2+\text{a}^2}-\sqrt{\text{x}^2+\text{b}^2}\big)}{\big(\sqrt{\text{x}^2+\text{a}^2}-\sqrt{\text{x}^2+\text{b}^2}\big)}\Bigg]$ $=\lim\limits_{\text{x}\rightarrow\infty}\Bigg[\frac{\big(\sqrt{\text{x}^2+\text{a}^2}-\sqrt{\text{x}^2+\text{b}^2}\big)\big(\sqrt{\text{x}^2+\text{a}^2}-\sqrt{\text{x}^2+\text{b}^2}\big)\big(\sqrt{\text{x}^2+\text{c}^2}-\sqrt{\text{x}^2+\text{d}^2}\big)}{\big(\sqrt{\text{x}^2+\text{c}^2}-\sqrt{\text{x}^2+\text{d}^2}\big)\big(\sqrt{\text{x}^2+\text{c}^2}+\sqrt{\text{x}^2+\text{d}^2}\big)\big(\sqrt{\text{x}^2+\text{a}^2}+\sqrt{\text{x}^2+\text{b}^2}\big)}\Bigg]$ $=\lim\limits_{\text{x}\rightarrow\infty}\frac{\big(\text{x}^2+\text{a}^2\big)-\big(\text{x}^2+\text{b}^2\big)}{\big(\text{x}^2+\text{c}^2\big)-\big(\text{x}^2+\text{d}^2\big)}\times\bigg(\frac{\sqrt{\text{x}^2+\text{c}^2}+\sqrt{\text{x}^2+\text{d}^2}}{\sqrt{\text{x}^2+\text{a}^2}+\sqrt{\text{x}^2+\text{b}^2}}\bigg)$ $=\lim\limits_{\text{x}\rightarrow{\infty}}\Big(\frac{\text{a}^2-\text{b}^2}{\text{c}^2-\text{d}^2}\Big)\bigg(\frac{\sqrt{\text{x}^2+\text{c}^2}+\sqrt{\text{x}^2+\text{d}^2}}{\sqrt{\text{x}^2+\text{a}^2}+\sqrt{\text{x}^2+\text{b}^2}}\bigg)$ Dividing the numerator and the denominator by x: $=\lim\limits_{\text{x}\rightarrow{\infty}}\Big(\frac{\text{a}^2-\text{b}^2}{\text{c}^2-\text{d}^2}\Big)\begin{pmatrix}\frac{\sqrt{1+\frac{\text{a}^2}{\text{x}^2}}+\sqrt{1+\frac{\text{a}^2}{\text{x}^2}}}{\sqrt{1+\frac{1}{\text{a}^2}}+\sqrt{1+\frac{\text{b}^2}{\text{a}^2}}}\end{pmatrix}$ $\text{As x}\rightarrow\infty,\frac{1}{\text{x}},\frac{1}{\text{x}^2}\rightarrow0$ $=\Big(\frac{\text{a}^2-\text{b}^2}{\text{c}^2-\text{d}^2}\Big)\Big(\frac{\sqrt{1}+\sqrt{1}}{\sqrt{1}+\sqrt{1}}\Big)$ $=\frac{\text{a}^2-\text{b}^2}{\text{c}^2-\text{d}^2}$

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