31 x 15 x 72 x 4 - 31 x 49 x 40
= 31 x 15 x 49 x 4 - 31 x 49 x 4 x 10
= 31 x 49 x 4 x ( 15 - 10)
= 31 x 49 x 4 x 5
...
31 x 15 x 7^2 x 4 - 31 x 49 x 40
= 31 x 15 x 49 x 4 - 31 x 46 x 40
= 31 x 49 x ( 15 x 4 - 40 )
= 31 x 49 x 20
= 30380
31 x 15 x 72 x 4 - 31 x 49 x 40
= 1860 x 49 - 1240 x 49
= (1860 - 1240) x 49
= 620 x 49
= 30380
31 x 15 x 72 x 4 - 31 x 49 x 40
= 1860 x 49 - 1240 x 49
= (1860 - 1240) x 49
= 620 x 49
= 30380
hok tốt
31x 15 x 72 x4 - 31 x49 x40
=31x15x49x4-31x49x4x10
=31x49x4x(15-10)
=31x49x4x5
=30380