6 . A closed rectangular box has length 4x cm , width 2x cm and height y cm , where x , y � 0. It is given that the sum of the length and the height of the rectangular box is 20 cm .
( a ) Write down
( i ) an expression for y in terms of x ;
( ii ) the possible range of values of x .
( b ) Express V in terms of x , where box .
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V cm is the volume of the rectangular
( c ) Using the graphic display calculator to find its maximum volume .
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