A= 1/1×2+1/3×4+…+1/199×200 B=1/101+1/102+…+1/200 Tính A/B

A= 1/1×2+1/3×4+…+1/199×200
B=1/101+1/102+…+1/200
Tính A/B

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  1. A=1/1×2+1/3×4+…+1/199×200.

    A=1-1/2+1/2-1/3+1/3-1/4+…+1/99-1/100

    A=1-1/100

    A=99/100

    B=1/101+1/102+…+1/200

    B=(1/101+1/102+…+1/120)+(1/121+1/122+..+1/150)+(1/151+1/180)+(1/181+…+1/200)

    B=20.1/120+30.1/150+30.1/180+20,1/200

    B=99/100

    Vậy A/B=

    99/100 phần 99/100

    A/B=1

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  2. A = $\frac{1}{1.2}$ + $\frac{1}{3.4}$ + …. + $\frac{1}{199.200}$ 

        = 1 – $\frac{1}{2}$ + $\frac{1}{3}$ – $\frac{1}{4}$ + …. + $\frac{1}{199}$ – $\frac{1}{200}$ 

        = (1 + $\frac{1}{2}$ + $\frac{1}{3}$ + $\frac{1}{4}$ + …. + $\frac{1}{199}$ + $\frac{1}{200}$) – 2($\frac{1}{2}$ + $\frac{1}{4}$ + …. + $\frac{1}{200}$)

        = (1 + $\frac{1}{2}$ + $\frac{1}{3}$ + $\frac{1}{4}$ + …. + $\frac{1}{199}$ + $\frac{1}{200}$) – (1 + $\frac{1}{2}$ + …. + $\frac{1}{100}$

        = $\frac{1}{101}$ + $\frac{1}{102}$ + …. + $\frac{1}{200}$ 

    B = $\frac{1}{101}$ + $\frac{1}{102}$ + …. + $\frac{1}{200}$

    $\frac{A}{B}$ =  $\frac{\frac{1}{101} + \frac{1}{102} + …. + \frac{1}{200}}{\frac{1}{101} + \frac{1}{102} + …. + \frac{1}{200}}$ = 1

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