Đáp án:
a, -1/3 -4/7
= ( -7-12)/21= -19/21
b,45/26×-13/9
= (45× -13)/ (26×9)
=-5/2
c, -5/8 : 7/5
= -5/8 ×5/7= -25/56
d, -3/4.5+ 3/5.6+...+ 3/49.50
= 3 ( 1/4.5+ 1/5.6 +...+ 49.50)
=3( 1/4-1/5+1/5-...+1/49-1/50)
=3 (1/4-1/50)= 3. 23/100 =69/100
e, 1/9.11+ 1/11.13+...+ 1/99.101
= 1/2 .( 2/9.11+ 2/11.13+...+ 2/99.101)
=1/2.( 1/9-1/11+...+1/99-1/101)
=1/2( 1/9-1/101)
=1/2 . 92/909= 46/909
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