To find a missing side in a right rtaingle as given use Pythagorean theorem:
[tex]hypotenuse^2=Leg1^2+Leg2^2[/tex]
Misising side in given triangle is a Leg:
[tex]\begin{gathered} Subtract\text{ Leg2}^2\text{ from both sides of the equation:} \\ hypotenuse^2-Leg2^2=Leg1^2+Leg2^2-Leg2^2 \\ hypotenuse^2-Leg2^2=Leg1^2 \\ \\ Rewrite_ \\ Leg1^2=hypotenuse^2-Leg2^2 \\ Take\text{ square root of both sides of the equation:} \\ \sqrt{Leg1^2}=\sqrt{hypotenuse^2-Leg2^2} \\ Leg1=\sqrt{hypotenuse^2-Leg2^2} \end{gathered}[/tex]
[tex]Leg1=\sqrt{hypotenuse^2-Leg2^2}[/tex][tex]\begin{gathered} x=\sqrt{21^2-10^2} \\ x=\sqrt{441-100} \\ x=\sqrt{341} \\ x\approx18.47 \end{gathered}[/tex]Then, the approximate value of x is 18.47Answer: A
