Question 179419
I'm assuming that the missing side is the hypotenuse.



So we basically have this triangle set up:



{{{drawing(500,500,-0.5,2,-0.5,3.2,
line(0,0,0,3),
line(0,3,2,0),
line(2,0,0,0),
locate(-0.2,1.5,9),
locate(1,-0.2,12),
locate(1,2,x)
)}}}



Since the legs are {{{9}}} and {{{12}}} this means that {{{a=9}}} and {{{b=12}}}


   

Also, since the hypotenuse is {{{x}}}, this means that {{{c=x}}}.



{{{a^2+b^2=c^2}}} Start with the Pythagorean theorem.



{{{9^2+12^2=x^2}}} Plug in {{{a=9}}}, {{{b=12}}}, {{{c=x}}} 



{{{81+12^2=x^2}}} Square {{{9}}} to get {{{81}}}.



{{{81+144=x^2}}} Square {{{12}}} to get {{{144}}}.



{{{225=x^2}}} Combine like terms.



{{{x^2=225}}} Rearrange the equation.



{{{x=sqrt(225)}}} Take the square root of both sides. Note: only the positive square root is considered (since a negative length doesn't make sense).



{{{x=15}}} Simplify the square root.



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Answer:



So the solution is {{{x=15}}} which means that the length of the missing side is 15 yards.