 −  −  −   −  +  +  +   +           A/...

1.3 2.4 3.5 17.19

21 28 36 1326

HD:

a, Ta có:

4 9 16 17.19 1 2.2 3.3 4.4 18.18

= = 36

. . .... . . ...

A = + = ( )( )

( 2.3.4...18 2.3.4...18 )( ) 18.2

= 19