6/13*17-12-3/13
=3/13*2*17-12-3/13
=3/13*(34-1)-12
=99/13-156/13
=-57/13
=-4又6/13
8/17+8/13]*13/8*17/8
=8/17*13/8*17/8+8/13*13/8*17/8
=13/8+17/8
=30/8
=3又3/4