This model represents the expression .
1
+
1
+
1
+
1
+
x
+
x
1
1
1
1
x
x
Which expression is equivalent to ?
1
+
1
+
1
+
1
+
x
+
x
6
+
x
2
+
4
x
4
+
2
x
6
x
ref_doc_title.
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