Bjr calculer astucieusement : 2/15 -[ 7/13-(20/13 - 17/15 ) -1 ]

2/15 - [7/13 - (20/13 - 17/15 ) -1 ]

= 2/15 - (7/13 - 20/13 + 17/15 - 1)

= 2/15 - 7/13 + 20/13 - 17/15 + 1

= 2/15 - 17/15 - 7/13 + 20/13 + 1

= -15/15 + 13/13 + 1

= -1 + 1 + 1

= 1