Surface Area of Rectangular Solid

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What is the surface area of a certain rectangular solid?

(1) Two adjacent faces of the solid have areas 30 and 45, respectively.

(2) The volume of the solid is 270.

Review: Surface Area of Rectangular Solid




Explanation

To find the surface area of this rectangular solid, we will most likely require the height, width, and depth of the shape. We can call the three dimensions, a, b, and c, so that the surface area would be . On to the statements, separately first.

Statement (1) tells us about two adjacent faces. They have one dimension in common, which means they have one variable in common, which we can take to be a. We can interpret this as and . That gives us a total of two equations and three variables, so we cannot solve directly. Meanwhile, we don't know . We can imagine a case in which . Then . Case II: Then , and since the two cases have different bc's and equal other areas, they have different surface areas. Insufficient.

Statement (2) tells us that . That's , so we could have . In another case, we could have . In one case, half the surface area is In the other, half the surface area is and we will just stop there because it is already greater, so we have two possibilities. Statement (2) alone is insufficient.

When we combine the statements, we now have three equations and three variables. We will be able to solve completely, and the single possible case turns out to be Case I above, which is not a complete surprise as it was allowed by both statements.

The correct answer is (C).


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