2/12×14+2/14×16+2/16×18+2/18×20+1/20.
问题描述:
+2 12×14
+2 14×16
+2 16×18
+2 18×20
. 1 20
答
+2 12×14
+2 14×16
+2 16×18
+2 18×20
,1 20
=
-1 12
+1 14
-1 14
+1 16
-1 16
+1 18
-1 18
+1 20
,1 20
=
.1 12