21. If $f ( x ) = \int _ { x } { } ^ { 2 ( x 2 + 1 ) } e ^ { - t 2 } d t$, then $f ( x )$ increases in: (a) $\quad ( 2,2 )$ (b) no value of $x$ (c) $\quad ( 0 , ¥ )$ (d) $( - ¥ , 0 )$
A tangent is drawn to the ellipse $\frac { x ^ { 2 } } { 27 } + y ^ { 2 } = 1$, at the point $( 3 \sqrt { 3 } \cos \theta , \sin \theta )$, where $0 < \theta < \pi / 2$. The sum of the intercepts of the tangent with the coordinate axes is least when $\theta$ equals
21. If $f ( x ) = \int _ { x } { } ^ { 2 ( x 2 + 1 ) } e ^ { - t 2 } d t$, then $f ( x )$ increases in:\\
(a) $\quad ( 2,2 )$\\
(b) no value of $x$\\
(c) $\quad ( 0 , ¥ )$\\
(d) $( - ¥ , 0 )$\\