Question
Four digit numbers are formed using the digits $1 , 2 , 3, 4$ (repetition is allowed). The number of such four digit numbers divisible by $11$ is
$x+z=y+w=k$
$\mathrm{k}=2: \quad 1^{2}$
$\mathrm{k}=3: \quad 2^{2}$
$k=4: \quad 3^{2}$
$k=5: \quad 4^{2}$
$k=6: \quad 3^{2}$
$k=7: \quad 2^{2}$
$k=8: \quad 1^{2}$
$\Rightarrow$ Total $=44$
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$\sin ^{-1}\left(\sum_{i=1}^{\infty} x^{i+1}-x \sum_{i=1}^{\infty}\left(\frac{x}{2}\right)^i\right)=\frac{\pi}{2}-\cos ^{-1}\left(\sum_{i=1}^{\infty}\left(-\frac{x}{2}\right)^i-\sum_{i=1}^{\infty}(-x)^i\right)$
lying in the interval $\left(-\frac{1}{2}, \frac{1}{2}\right)$ is. . . . .
(Here, the inverse trigonometric functions $\sin ^{-1} x$ and $\cos ^{-1} x$ assume values in $\left[-\frac{\pi}{2}, \frac{\pi}{2}\right]$ and $[0, \pi]$, respectively.)