Question

\[\int\limits_1^3 \left( 2x + 3 \right) dx\]

Answer 1

\[\int_a^b f\left( x \right) d x = \lim_{h \to 0} h\left[ f\left( a \right) + f\left( a + h \right) + f\left( a + 2h \right) . . . . . . . . . . . . . . . + f\left( a + \left( n - 1 \right)h \right) \right]\]
\[\text{where }h = \frac{b - a}{n}\]

\[\text{Here, }a = 1, b = 3, f\left( x \right) = 2x + 3, h = \frac{3 - 1}{n} = \frac{2}{n}\]

Therefore,

\[I = \int_1^3 \left( 2x + 3 \right) d x\]
\[ = \lim_{h \to 0} h\left[ f\left( 1 \right) + f\left( 1 + h \right) + . . . . . . . . . . . . . . . . . . . . + f\left\{ 1 + \left( n - 1 \right)h \right\} \right]\]
\[ = \lim_{h \to 0} h\left[ \left( 2 + 3 \right) + \left( 2 + 2h + 3 \right) + . . . . . . . . . . . . . . . + \left\{ 2 + 2\left( n - 1 \right)h + 3 \right\} \right]\]
\[ = \lim_{h \to 0} h\left[ 5n + 2h\left\{ 1 + 2 + 3 . . . . . . . . . + \left( n - 1 \right) \right\} \right]\]
\[ = \lim_{h \to 0} h\left[ 5n + 2h\frac{n\left( n - 1 \right)}{2} \right]\]
\[ = \lim_{n \to \infty} \frac{2}{n}\left[ 5n + 2n - 2 \right]\]
\[ = \lim_{n \to \infty} 2\left( 7 - \frac{2}{n} \right)\]
\[ = 14\]