Bài 1
A= 4/7+3/4+2/7+5/4+1/7
A=(4/7+2/7+1/7)+(3/4+5/4)
A=1+2
A=3
B= -4/12 + 18/45 + -6/9 + -21/35 + 6/30
B= -1/3 + 2/5 + -2/3 + -3/5 + 1/5
B= (-1/3 + -2/3)+ (2/5 + -3/5 + 1/5)
B= -1 + 0
B= -1
Bài 2
A = 31/17 + -5/13 + (-8/13) - 14/17
A = 31/17 + -5/13 + (-8/13) + (-14/17)
A = [31/17 + (-14/17)] + [ -5/13 + (-8/13)
A = 1 + (-1)
A = 0