Answer:
c =
km
Step-by-step Step-by-step explanation:
Here's the required formula to find the missing side of triangle by pythagoras theorem :
![{\longrightarrow{\pmb{\sf{{(a)}^(2) + {(b)}^(2) = {(c)}^(2)}}}}](https://img.qammunity.org/2023/formulas/mathematics/college/kxzaspsc2glouo5hs58qqh4dhn1bj0kt0l.png)
a = 20 km
b = 21 km
c = ?
Substituting all the given values in the formula to find the third side of triangle :
![\begin{gathered} \quad{\longrightarrow{\sf{{(a)}^(2) + {(b)}^(2) = {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{{(20)}^(2) + {(21)}^(2) = {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{(20 * 20) + (21 * 21) = {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{(400) + (441) = {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{400 + 441 = {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{841= {(c)}^(2)}}} \\ \\ \quad{\longrightarrow{\sf{c = √(841)}}} \\ \\ \quad{\longrightarrow{\sf{\underline{\underline{\purple{c = 29 \: km}}}}}}\end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/8krrbc2a9rbq70cul9544632lfsnrlt7vr.png)
Hence, the length of missing side of triangle is 29 km.
![\rule{300}{2.5}](https://img.qammunity.org/2023/formulas/mathematics/high-school/4wr5wvuxv3n1yp1b1sykhxf527peu2ltxm.png)