The sum of the length breadth and wide depth of a cuboid is 20cm and it's diagonal is 4√5cm then it's surface area
(a) 400
(b) 420
(c) 320
(d) 300
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We can use Pythagoras in three-dimensions
a^2 + b^2 + c^2 = (4 sqrt5)^2 = 80
Also
a+b+c = 20
Square it
a^2 + b^2 + c^2 + 2ab + 2bc + 2ac = 400
Subtract
2ab + 2bc + 2ac = 400-80
=320
ANSWER:-(C) 320
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