Find the volume $ V $ of a pyramid if side $a = 20$ and slant height $s = 25$.

$V = \frac{ 2000 \sqrt{ 21}}{ 3 }$

STEP 1: find height $ h $

To find height $ h $ use Pythagorean Theorem:

$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$

After substituting $ a = 20 $ and $ s = 25 $ we have:

$$ h ^ {\,2} + \frac{ 20^2 }{ 4 } = 25^2 $$ $$ h ^ {\,2} + \frac{ 400 }{ 4 } = 25^2 $$ $$ h ^ {\,2} + 100 = 25^2 $$ $$ h ^ {\,2} = 625 - 100 $$ $$ h ^ {\,2} = 525 $$ $$ h = \sqrt{ 525 } $$$$ h = 5 \sqrt{ 21 } $$

STEP 2: find volume $ V $

To find volume $ V $ use formula:

$$ V = \dfrac{ a ^{ 2 } \cdot h }{ 3 } $$

After substituting $ a = 20 $ and $ h = 5 \sqrt{ 21 } $ we have:

$$ V = \dfrac{ 400 \cdot 5 \sqrt{ 21 } }{ 3 }$$$$ V = \dfrac{ 2000 \sqrt{ 21 } }{ 3 } $$$$ V = \frac{ 2000 \sqrt{ 21}}{ 3 } $$

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