A=1/2(1/1.2-1/2.3+1/2.3-1/3.4+...............+1/37.38-1/38.39)
A=1/2(1/1.2-1/38.39)
A=1/2.370/741
A=185/741
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Tính A=\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+...+\dfrac{1}{37.38.39}\)
Tính tổng A= 1/1.2.3 +1/2.3.4+1/3.4.5+.......+1/37.38.39
1\1.2.3+1\2.3.4+..................+1\37.38.39=
1/1.2.3+1/2.3.4+1/3.4.5+...+1/37.38.39
1/1.2.3+1/2.3.4+1/3.4.5+....+1/37.38.39 = ?
1/1.2.3+1/2.3.4+1/3.4.5+...+1/37.38.39
1/1.2.3+1/2.3.4+1/3.4.5+...+1/37.38.39
Tính nhanh\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{37.38.39}\)
Tính tổng:
\(B=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{37.38.39}\)