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Geometry

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Junior | Next Rank: 30 Posts
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by Shamhoon » Wed Mar 09, 2011 8:14 am
A pyramid with four equal-sized flat surfaces and a base of 36 feet (square) has a height of 10 feet. What is the total

surface area of the pyramid, excluding the base?

A . 12 root of 31

B. 12 root of 109

C. 24 root of 31

D. 24 root of 109

E. 48 root of 31
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by navami » Wed Mar 09, 2011 9:02 am
IMO B.

from base area, we can calculate the base sides,
for a square area36, each side: a=square root of 36=6

Consider the right triangle where, base=6/2=3 and height given=10,
then by Pythagoras theorem hypotnuse= square root of (10^2 + 3^2) = square root of 109

now surface area of each side( triangle) = (1/2)*base*height = (1/2)*6*square root of 109

since there are four sides, hence total surface area= 4*(1/2)*6*square root of 109 = Option B
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by Shamhoon » Wed Mar 09, 2011 9:14 am
thank you so much for the reply...

But why are we not taking 10 as the height ???
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by navami » Wed Mar 09, 2011 9:35 am
Pyramid is slant, the height (10) given in the problem is the distance between the pyramid's topmost point to the ground.
so to calculate the surface area u need the slant height of each of the triangles (each of the four sides), which can be calculated from Pythagorean theorem.

Please Ref to the attached diagram.
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by Shamhoon » Wed Mar 09, 2011 10:29 am
thanks a ton!

very well explained:-)
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