Question: help with u substitution

What is the value of u1 in this u substitution to go from -2*Pi*(int(sqrt(-3*u^2+4), u = 1 .. 0)) to -2*Pi*(int((4/3)*sqrt(3)*cos(u1)^2, u1 = (1/3)*Pi .. 0))? I'm just not seeing it.

 

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