When π/2

√(x？ - 1)=√[( 1/cos？ t)- 1]=√[(sin？ t)/(cos？ T)]=(sint)/∣cost∣, because when 0≦t≦π, sint≧0 remains unchanged.

Therefore, it can have no absolute value sign; The cost should be signed, so the absolute value should be signed.

In the later integral, ∫{[√(x? - 1)]/x？ }dx=∫[(sin？ tcos？ T)/∣cost∣]dt, in order to facilitate integration, here he removed the absolute value sign of cost and replaced it with sgnx, and got ∫sin? tcostdt？ sgnx=( 1/3)sin？ t？ Sgnx+C indicates that there is a symbol selection problem here. The selection rules of symbols are the rules mentioned above. Sgnx is called a symbolic function.