如題 最近解題目遇到這題
 
Ll=8  L2=10  L3=12
M12=2
M13=3
M23=1

他答案給的是Lab=2.787 但我怎麼都解不出這個答案 請各位高手幫忙一下

 A _______________________
                    | *               |*           |
                    |                 |            |
                    |                 |            |
                    |                 |            |
                  L1             L2          L3
                    |                 |            |
                    |                 |            |
                    |                 |            |
                    |                 |            |*
 B _______________________