Step-by-step explanatii just took the test and it was right. the base would be 20 and the height would be 10. from there you just you the formula for area of a right triangle.
Suppose first that is a normal subgroup. Then by definition we must have for all , for every . Let and choose (). By hypothesis we have , i.e. for some , thus . So we have . You can prove in the same way.
Suppose for all . Let , we have to prove for every . So, let . We have that for some (by the hypothesis). hence we have . Because was chosen arbitrarily we have the desired
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