1, tinh hop li A=2^2/3.5+2^2/5.7+2^2/7.9+…+2^2/69.71
2, chung to rang A=1n+3/21n+5 la phan so toi gian
giup minh minh dang can gap
1, tinh hop li A=2^2/3.5+2^2/5.7+2^2/7.9+…+2^2/69.71
2, chung to rang A=1n+3/21n+5 la phan so toi gian
giup minh minh dang can gap
1.
A= 2^2/3.5+2^2/5.7+2^2/7.9+…+2^2/69.71
=4/3.5=4/5.7+4/7.9+…+4/69.71
=4/2.(2/3.5+2/5.7+2/7.9+…+2/69.71
=4/2.(1/3-1/5+1/5-1/7+1/7-1/9+…+1/69-1/71)
=4/2.(1/3-1/71)
=4/2.68/213
=136/213
Đáp án:
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