If $(\operatorname{cosec} A+\cot A)=m$ then prove that $\frac{m^{2}-1}{m^{2}+1}=\cos \theta$.
Given: $\operatorname{cosec} A+\cot A=m$ ....(1)
We know
$\operatorname{cosec}^{2} A-\cot ^{2} A=1$
$\Rightarrow(\operatorname{cosec} A-\cot A)(\operatorname{cosec} A+\cot A)=1 \quad\left[a^{2}-b^{2}=(a-b)(a+b)\right]$
$\Rightarrow(\operatorname{cosec} A-\cot A) m=1 \quad[$ From $(1)]$
$\Rightarrow \operatorname{cosec} A-\cot A=\frac{1}{m} \quad \ldots \ldots(2)$
Adding (1) and (2), we get
$\operatorname{cosec} A+\cot A+\operatorname{cosec} A-\cot A=m+\frac{1}{m}$
$\Rightarrow 2 \operatorname{cosec} A=\frac{m^{2}+1}{m}$
$\Rightarrow \operatorname{cosec} A=\frac{m^{2}+1}{2 m}$
$\Rightarrow \frac{1}{\operatorname{cosec} A}=\frac{2 m}{m^{2}+1}$
$\Rightarrow \sin A=\frac{2 m}{m^{2}+1} \quad \ldots .(3)$
Subtracting (2) from (1), we get
$\operatorname{cosec} A+\cot A-\operatorname{cosec} A+\cot A=m-\frac{1}{m}$
$\Rightarrow 2 \cot A=\frac{m^{2}-1}{m}$
$\Rightarrow \cot A=\frac{m^{2}-1}{2 m} \quad \ldots \ldots(4)$
Now,
$\cos A=\sin A \times \cot A$
$\Rightarrow \cos A=\frac{2 m}{m^{2}+1} \times \frac{m^{2}-1}{2 m} \quad[$ From $(3)$ and $(4)]$
$\Rightarrow \cos A=\frac{m^{2}-1}{m^{2}+1}$