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Aus Online Mathematik Brückenkurs 2

If we set u=2x+3 , the integral simplifies to eu. However, this is only part of the truth. We must in addition take account of the relation between the integration element dx and du, which can give undesired effects. In this case, however, we have

du=2x+3dx=2dx 

which only affects by a constant factor, so the substitution u=2x+3 seems to work, in spite of everything:

120e2x+3dx=u=2x+3du=2dx=2143eudu=21eu43=21e4−e3

NOTE: Another possible substitution is u=e2x+3 which also happens to work (usually, such an extensive substitution almost always fails).