Question

Tính giá trị biểu thức: 

A = 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132

Answer 1

A=1/30 + 1/42 + ...+ 1/132

   =1/5.6 + 1/6.7 +1/7.8 +....+1/11.12

   =6-5/5.6 + 7-6/6.7+ ...+ 12-11/11.12

   =1/5 -1/6 +1/6 - 1/7+....+ 1/11 - 1/12

   =1/5-1/12

   =7/60

Answer 2

A= 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9 + 1/9x10 + 1/10x11 + 1/11x12

A= 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 + 1/11 - 1/12

A= 1/5 - 1/12 = 7/60

Answer 3

A = 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132

A = 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 + 1/10.11 + 1/11.12

A = 1/5 - 1/6 + 1/6 - 1/7 +......+1/11 - 1/12

A = 1/5 - 1/12

A =...........Tự mà tính nha!