[tex] \frac{1}{f}= \frac{1}{x_2}- \frac{1}{x_1} [/tex]
[tex]a.) \frac{2\cdot1}{12\cdot2}= \frac{1}{x_2}- \frac{1}{24} \longrightarrow \frac{2}{24}+ \frac{1}{24}= \frac{1}{x_2}\longrightarrow \frac{1}{x_2}= \frac{3}{24}\longrightarrow \frac{1}{x_2}= \frac{1}{8} [/tex]
[tex]\boxed{\boxed{x_2=8}}[/tex]
b.) [tex]\underbrace{\frac{1}{12}+\frac{1}{13}}= \frac{1}{x_2} \longrightarrow \frac{13+12}{156}= \frac{1}{x_2}\longrightarrow \frac{25}{126}= \frac{1}{x_2}\longrightarrow \boxed{x_2 = \frac{126}{25} }[/tex]
c.) [tex]\underbrace{ \frac{1}{12}+ \frac{1}{6}}= \frac{1}{x_2}\longrightarrow \frac{1+2}{12}= \frac{1}{x_2} \longrightarrow \frac{3\div3}{12\div3}= \frac{1}{x_2}\longrightarrow \frac{1}{x_2}= \frac{1}{4} [/tex]
[tex]\boxed{\boxed{x_2=4}}[/tex]
