Question 1085741
C is the set of real x such that {{{ x < 3 }}}
That's everything to the left of the 3 below, excluding the 3 itself:
{{{ 
 number_line( 600, -10, 10,  3)
}}}
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D is the set of real x such that {{{ x <= 9 }}} 
That's everything to the left  of the 9 below, including the 9 itself:
{{{
 number_line( 600, -10,  10,  9)

}}}

{C} U {D} is the region to the left of the 9, including the 9 itself.   Since D includes C, the union of the two sets coincides with D.  Using interval notation, it would be written ( {{{ -infinity }}} , {{{ 9 }}} ]    (open paren's means that endpoint is excluded, square bracket means that endpoint is included).

{C} intersect {D} is the region to the left of 3 (excluding the 3 itself) and coincides with set C.   In interval notation:  ( {{{-infinity }}}, {{{ 3 }}} )

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