MCQ
${d \over {dx}}[(1 + {x^2}){\tan ^{ - 1}}x] = $
- A$x\,{\tan ^{ - 1}}x$
- B$2\,{\tan ^{ - 1}}x$
- ✓$2x\,{\tan ^{ - 1}}x + 1$
- D$x\,{\tan ^{ - 1}}x + 1$
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$x-2 y+5 z=0$
$-2 x+4 y+z=0$
$-7 x+14 y+9 z=0$
such that $15 \leq x^{2}+y^{2}+z^{2} \leq 150 .$ Then, the number of elements in the set $S$ is equal to