A Gaussian integral with Grassmann variables can be written as follows: $$\int\prod_{i=1}^n (d\vartheta_i\,d\bar{\vartheta}_i)\; \exp{\left(\sum_{i,j=1}^n \bar{\vartheta}_i\,a_{ij}\,\vartheta_j\right)}$$ The book I'm studying on goes on expanding the integrand at the following manner: $$\exp{\left(\sum_{i,j=1}^n \bar{\vartheta}_i\,a_{ij}\,\vartheta_j\right)} = \prod_{i=1}^n \exp{\left(\bar{\vartheta}_i \sum_{j=1}^n a_{ij}\,\vartheta_j\right)} = \prod_{i=1}^n \left(1+\bar{\vartheta}_i \sum_{j=1}^n a_{ij}\,\vartheta_j\right)$$ While I'm able to understande the first passage, I really can't figure out the last one; any help would be much apprecited!
EDIT: @Nox's comment helped me figure out that it's just a Taylor expansion (and it gives that result because $\vartheta_i \vartheta_i = 0$, being a Grassmann variable.