Definite Integral of Linear Function

In this tutorial we shall find an example of definite integral of linear function from limits 1 to 2.

The integration of the form

I =  \int\limits_1^2 {\left( {4x + 1} \right)dx}


We using the basic rule of definite integral \int\limits_a^b {f\left( x \right)dx = \left|  {F\left( x \right)} \right|_a^b}  =  \left[ {F\left( b \right) - F\left( a \right)} \right], we have

\begin{gathered} I = 4\int\limits_1^2 {xdx}  + \int\limits_1^2 {1dx} \\ \Rightarrow \int\limits_1^2 {\left( {4x + 1}  \right)dx}  = 4\left| {\frac{{{x^2}}}{2}}  \right|_1^2 + \left| x \right|_1^2 \\ \Rightarrow \int\limits_1^2 {\left( {4x + 1}  \right)dx}  = 2\left| {{x^2}} \right|_1^2  + \left| x \right|_1^2 \\ \Rightarrow \int\limits_1^2 {\left( {4x + 1}  \right)dx}  = 2\left[ {{{\left( 2  \right)}^2} - {{\left( 1 \right)}^2}} \right] + \left[ {2 - 1} \right] \\ \Rightarrow \int\limits_1^2 {\left( {4x + 1}  \right)dx}  = 2\left[ {4 - 1} \right] +  \left[ {2 - 1} \right] \\ \Rightarrow \int\limits_1^2 {\left( {4x + 1}  \right)dx}  = 7 \\ \end{gathered}