area under a curve

Calculus Level 2

calculate the area under a curve y = x 2 + 4 x 3 y=x^{2}+4x-3 between x = 2 x=2 and x = 5 x=5 don't consider the unit.


The answer is 72.

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1 solution

Ikkyu San
Aug 3, 2015

The area under a curve of above equation is 2 5 ( x 2 + 4 x 3 ) d x \displaystyle\int_2^5(x^2+4x-3)\ \mathrm dx

From,

( x 2 + 4 x 3 ) d x = x 2 d x + 4 x d x 3 d x = x 3 3 + 4 ( x 2 2 ) 3 x + c = x 3 3 + 2 x 2 3 x + c \begin{aligned}\displaystyle\int(\color{#D61F06}{x^2}+\color{#20A900}{4x}-\color{#3D99F6}3)\ \mathrm dx=&\ \color{#D61F06}{\displaystyle\int x^2\ \mathrm dx}+\color{#20A900}{4\displaystyle\int x\ \mathrm dx}-\color{#3D99F6}{\displaystyle\int3\ \mathrm dx}\\=&\ \color{#D61F06}{\dfrac{x^3}3}+\color{#20A900}{4\left(\dfrac{x^2}2\right)}-\color{#3D99F6}{3x}+c\\=&\ \color{#D61F06}{\dfrac{x^3}3}+\color{#20A900}{2x^2}-\color{#3D99F6}{3x}+c\end{aligned}

Thus,

2 5 ( x 2 + 4 x 3 ) d x = ( x 3 3 + 2 x 2 3 x ) 2 5 = ( 5 3 3 + 2 ( 5 ) 2 3 ( 5 ) ) ( 2 3 3 + 2 ( 2 ) 2 3 ( 2 ) ) = 125 3 + 50 15 8 3 8 + 6 = 39 + 33 = 72 \begin{aligned}\displaystyle\int_\color{magenta}2^{\color{#302B94}5}(\color{#D61F06}{x^2}+\color{#20A900}{4x}-\color{#3D99F6}3)\ \mathrm dx=&\ \left.\left(\color{#D61F06}{\dfrac{x^3}3}+\color{#20A900}{2x^2}-\color{#3D99F6}{3x}\right)\right|_{\color{magenta}2}^{\color{#302B94}5}\\=&\ \left(\dfrac{\color{#302B94}5^{\color{#D61F06}3}}{\color{#D61F06}3}+\color{#20A900}2\color{#302B94}{(5)}^{\color{#20A900}2}-\color{#3D99F6}3\color{#302B94}{(5)}\right)-\left(\dfrac{\color{magenta}2^{\color{#D61F06}3}}{\color{#D61F06}3}+\color{#20A900}2\color{magenta}{(2)}^{\color{#20A900}2}-\color{#3D99F6}3\color{magenta}{(2)}\right)\\=&\ \dfrac{125}3+50-15-\dfrac83-8+6\\=&\ 39+33=\boxed{72}\end{aligned}

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