s
=
v
1
t
1
+
v
2
t
2
+
v
3
t
3
=
u
(
t
1
+
t
2
+
t
3
)
s
=
v
1
t
1
+
v
2
t
2
+
v
3
t
3
=
u
(
t
1
+
t
2
+
t
3
)
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