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Integral result rearrangement.

$$\tag{1} \int \sin ax \ dx = - \frac{1}{a} \cos ax + c $$

$$\tag{2} \int_{x_1}^{x_2} \sin ax \ dx = \left[ - \frac{1}{a} \cos ax \right]_{x = x_1}^{x_2} $$

$$\tag{3} \begin{array}{rcl} \displaystyle \left[ - \frac{1}{a} \cos ax \right]{x = x_1}^{x_2} & = & \displaystyle \left( - \frac{1}{a} \cos ax_2 \right) - \left( - \frac{1}{a} \cos ax_1 \right) \newline & = & \displaystyle - \frac{1}{a} (\cos ax_2 - \cos ax_1) \newline & = & \displaystyle - \frac{1}{a} [\cos ax]{x = x_1}^{x_2} \end{array} $$

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