① 28+45+72
②1+3+5+7+9
③ 100-57-23
28+45+72
=28+72+45
=100+45
=145
1+3+5+7+9
=(1+9)+(3+7)+5
=10+10+5
=25
100-57-23
=100-(57+23)
(Ⅰ)求B;
(Ⅱ)若a=2 3 c,S△ABC=2 3 ,求b.