Question : The base of a right pyramid is an equilateral triangle of side $10\sqrt3$ cm. If the total surface area of the pyramid is $270\sqrt3$ sq. cm. its height is:
Option 1: $12\sqrt3$ cm
Option 2: $10$ cm
Option 3: $10\sqrt3$ cm
Option 4: $12$ cm
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Correct Answer: $12$ cm
Solution :
Given: The base of a right pyramid is an equilateral triangle. Side of a triangle $AB = 10\sqrt3$ cm The total surface area of a pyramid $= 270\sqrt3$ cm 2 $\therefore$ Inradius of a triangle $OE = \frac{\text{side of equilateral triangle}}{2\sqrt3}$ = $\frac{10\sqrt3}{2\sqrt3}=5$ cm Now, slant height (l) $= \sqrt {h^{2}+OE^{2}}$ $= \sqrt {h^{2}+25}$ Base area $=\frac{\sqrt{3}}{4}×(10\sqrt{3})^{2}$ Total surface area $= \frac{1}{2}×(\text{perimeter of the base×slant height)+base area}$ ⇒ $270\sqrt3=\frac{1}{2}(10\sqrt{3}×3×\sqrt {h^{2}+25})+\frac{\sqrt{3}}{4}×(10\sqrt{3})^{2}$ ⇒ $270\sqrt3=15\sqrt{3}(\sqrt{{h^{2}+25}})+75\sqrt{3}$ ⇒ $15\sqrt{3}(\sqrt{{h^{2}+25}})=195\sqrt3$ ⇒ $\sqrt{{h^{2}+25}}=13$ ⇒ $h^{2}=144$ ⇒ $h = 12$ cm Hence, the correct answer is $12$ cm.
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