Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2008/iii/paper-3/3/v/solution

The PDF's specified five-by-five diagonal matrix is , so and . In the root sum for the Killing form, the two short roots contribute , the two short roots contribute zero, and the four long roots each contribute one. Therefore
This computation uses the original PDF matrix; the converted TeX incorrectly gives its dimensions.

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