\(3+4+5+6+7=(3+7)+(4+6)+5=10+10+5=25\)
\(12+13+14+16+17+18=(12+18)+(13+17)+(14+16)=30+30+30=90\)
\(a,3+4+5+5+6+7\\ =\left(3+7\right)+\left(4+6\right)+\left(5+5\right)\\ =10+10+10\\ =30\\ b,12+13+14+16+17+18\\ =\left(12+18\right)+\left(13+17\right)+\left(14+16\right)\\ =30+30+30\\ =90\)
TimeSun
3
+
4
+
5
+
6
+
7
=
(
3
+
7
)
+
(
4
+
6
)
+
5
=
10
+
10
+
5
=
25
12
+
13
+
14
+
16
+
17
+
18
=
(
12
+
18
)
+
(
13
+
17
)
+
(
14
+
16
)
=
30
+
30
+
30
=
9
3
+
4
+
5
+
6
+
7
=
(
3
+
7
)
+
(
4
+
6
)
+
5
=
10
+
10
+
5
=
25
12
+
13
+
14
+
16
+
17
+
18
=
(
12
+
18
)
+
(
13
+
17
)
+
(
14
+
16
)
=
30
+
30
+
30
=
90
3 + 4 + 5 + 5 + 6 + 7 12 + 13 + 14 + 16 + 17 + 18
= ( 3 + 7 ) + ( 4 + 6 ) + ( 5 + 5 ) = ( 12 + 18 ) + ( 13 + 17 ) + ( 14 + 16 )
= 10 + 10 + 10 = 30 + 30 + 30
= 20 + 10 = 60 + 30
= 30 = 90
