Question -
Answer -
Let the side of the square to be cut offbe x cm. Then, the length and the breadth of the box will be(18 − 2x) cm each and the height of the box is x cm.
Therefore, the volume V(x)of the box is given by,
V(x)= x(18 − 2x)2
x = 9or x = 3
If x = 9, then the lengthand the breadth will become 0.
x ≠ 9.
x = 3.
Now, 
By second derivative test, x =3 is the point of maxima of V. Hence, if we remove a square of side 3 cm from each cornerof the square tin and make a box from the remaining sheet, then the volume ofthe box obtained is the largest possible.