Trong mặt phẳng \(Oxyz\), mặt phẳng nào dưới đây nhận \(\overrightarrow n = \left( {3;1; - 7} \right)\) là một vectơ pháp tuyến?    

Trả lời: 27

Ta có \(F\left( x \right) = \left\{ \begin{array}{l}{x^2} + 5x + {C_1}\;\;{\rm{khi}}\;x \ge 1\\{x^3} + 4x + {C_2}\;{\rm{khi}}\;x < 1\end{array} \right.\).

Vì \(F\left( 0 \right) = 2\) nên \(F\left( 0 \right) = {0^3} + 4.0 + {C_2} = 2 \Rightarrow {C_2} = 2\).

Vì \(F\left( x \right)\) liên tục nên \(\mathop {\lim }\limits_{x \to {1^ + }} F\left( x \right) = \mathop {\lim }\limits_{x \to {1^ - }} F\left( x \right) = F\left( 1 \right)\)\( \Leftrightarrow 6 + {C_1} = 7 \Rightarrow {C_1} = 1\).

Do đó \(F\left( x \right) = \left\{ \begin{array}{l}{x^2} + 5x + 1\;\;{\rm{khi}}\;x \ge 1\\{x^3} + 4x + 2\;{\rm{khi}}\;x < 1\end{array} \right.\).

Ta có \(F\left( { - 1} \right) = {\left( { - 1} \right)^3} + 4.\left( { - 1} \right) + 2 = - 3;F\left( 2 \right) = {2^2} + 5.2 + 1 = 15\).

Do đó \(F\left( { - 1} \right) + 2F\left( 2 \right) = - 3 + 2.15 = 27\).

Ta có: \(g\left( 3 \right) = \int\limits_0^3 {f\left( t \right){\rm{d}}t}  = \int\limits_0^1 {f\left( t \right){\rm{d}}t}  + \int\limits_1^2 {f\left( t \right){\rm{d}}t}  + \int\limits_2^3 {f\left( t \right){\rm{d}}t} \)

\( = \int\limits_0^1 {{\rm{2d}}t}  + \int\limits_1^2 {{\rm{2}}t{\rm{d}}t}  + \int\limits_2^3 {\left( {12 - 4t} \right){\rm{d}}t} \)

\( = \left. {2t} \right|_0^1 + \left. {{t^2}} \right|_1^2 + \left. {\left( {12t - 2{t^2}} \right)} \right|_2^3 = 7\).