Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/ii/paper-4/19i/a/i/solution

Applying the Clebsch-Gordan decomposition for SU2 gives
and
On the summand , interchange of the two tensor factors acts by ; this is the flip parity in the SU2 tensor square. The even- summands make the symmetric square, so

New to topics? Read the docs here!