{container5 → container6}/lib/main.dart

@@ -64,6 +64,7 @@

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borderRadius: BorderRadius.circular(borderRadius),

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),

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duration: _duration,

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+
                            curve: Curves.easeInOutBack,

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),

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),

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ElevatedButton(

The easeInOutBack constant is only one of many that you can
  pass for the curve parameter. Explore the
  list of curve constants to discover more ways
  to use curve to modify the look and feel of your animations.

Putting it all together