\"\"

\

The side lengths of a right triangle are \"a ; \"c.

\

Apply Pythagorean theorem: \"a

\

\"20              (Substitute 20 for a and 21 for b)

\

\"400            (Evaluate powers: \"20)\"\"

\

\"841                       (Add: \"400)

\

\"square                (Take square root each side)

\

\"\"                 (Write \"\")

\

\"c                           (Cancel sqaure and root)\"\"

\

The unknown length of \"c.