\(S = \int\limits_a^b {\left( {f\left( x \right) - g\left( x \right)} \right)dx} \)

\(S = \int\limits_a^b {\left( {g\left( x \right) - f\left( x \right)} \right)dx} \)

\(S = \int\limits_a^b {\left| {f\left( x \right) - g\left( x \right)} \right|dx} \)

\(S = \int\limits_a^b {\left| {f\left( x \right)} \right|dx}  - \int\limits_a^b {\left| {g\left( x \right)} \right|dx} \)