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Pick up certain terms in multiplication [closed]

Suppose I have the following expression: (2a + 3b)^2 when expanded, it gives 4a^2 + 9b^2 +6a*b I want to pick up only the terms involving a* b, that is the term 6a*b. Is there an automatic way to pick ...
3
votes
1answer
96 views

Simplifying a difficult trigonometric sum

As this video shows, $$\tan \left( \frac{3 \pi}{11} \right) + 4 \sin \left( \frac{2 \pi}{11} \right) = \sqrt{11} .$$ The direct and obvious methods for Mathematica to find this solution do not work: <...