[Solved] how to use onslide to apply underbrace beneath operation

user2161721 Asks: how to use onslide to apply underbrace beneath operation
I am trying to apply a simple transition in beamer, after writing the operation without having to repeat the equation every time. I try the code :

Code:
begin{frame}
frametitle{Exemples}
begin{exampleblock}{2.1.2.Exemples}
Effectuer les opérations suivantes:
 begin{multicols}{2} % two columns
 begin{itemize}[<+->]
 item<1-> $A =underbrace{(15 - 7)}- 6 + 3 $
 item<3-> $A = underbrace{8 - 6} + 3 $   
 item<4-> $A = underbrace{2 + 3} $ 
 item<5-> $A = 5 $ 
 columnbreak
 item<2-> $B =underbrace{(9 + 13)}- 7.2 - 1 $
 item<6-> $B = underbrace{22 - 7.2} - 1 $   
 item<7-> $B = underbrace{14.8 - 1} $ 
 item<8-> $B = 13.8 $
 end{itemize}  
 end{multicols}
 end{exampleblock}
 end{frame}

I am trying to make the equations A and B appear at first, let the students have a time of reflection, then show the correction step by step, first by showing the underbrace beneath each operation, then transition to display the next line $ A = 8 - 6 + 3 $ … so I use only<1-> but its not working, I think its a basic question, Now I already inserted the transition between item correctly and I get the result:


enter image description here

I am just not familiar with this onslide<1-> action yet, please help fix this problem ,

thanks for the help !

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