Question : If the sum of three dimensions and the total surface area of a rectangular box are 12 cm and 94 cm2 respectively, then the maximum length of a stick that can be placed inside the box is:
Option 1: $5\sqrt2$ cm
Option 2: $5$ cm
Option 3: $6$ cm
Option 4: $2\sqrt5$ cm
Correct Answer: $5\sqrt2$ cm
Solution :
Sum of three dimensions = $l + b + h$ = 12 cm
Total surface area of cuboidal box = $2(lb + bh + lh)$ = 94 cm
2
$(l + b + h)^2$ = $l^2 + b^2 + h^2 + 2(lb + bh + lh)$
⇒ $12^2$ = $l^2 + b^2 + h^2 + 94$
⇒ $l^2 + b^2 + h^2$ = $144-94$ = $50$
The maximum length of a rod that can be put inside the box
= diagonal of the cuboid
= $\sqrt{l^2 + b^2 + h^2}$
= $\sqrt{50}$
= $5\sqrt2$ cm
Hence, the correct answer is $5\sqrt2$ cm.
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