Toán tính nhanh; B= 1/9.10 -1/8.9- 1/7.8- …………… -1/2.3 -1/1.2 09/07/2021 By Harper tính nhanh; B= 1/9.10 -1/8.9- 1/7.8- …………… -1/2.3 -1/1.2
`B = 1/(9*10) – 1/(8*9) – 1/(7*8) – … – 1/(2*3) – 1/(1*2)` `B = – (-1/(9*10) + 1/(8*9) + 1/(7*8) + … + 1/(2*3) + 1/(1*2))` `B = – (1/(1*2) + 1/(2*3) + … + 1/(8*9) – 1/(9*10))` `B = – (1/1 – 1/2 + 1/2 – 1/3 + … + 1/8 – 1/9 – 1/(9*10))` `B = – (1 – 1/9 – 1/(9*10))` `B = – (8/9 – 1/9 + 1/10)` `B = -79/90` Trả lời
$B=$$\frac{1}{9×10}$$-$$\frac{1}{8×9}$ $-….-$ $\frac{1}{1×2}$ $B=-$$($$\frac{1}{8×9}$ $+….+$ $\frac{1}{1×2}$$-$$\frac{1}{9×10}$$)$ $B=-$($\frac{1}{1×2}$ $+….+$ $\frac{1}{8×9}$$-$$\frac{1}{9×10}$$)$ $B=-$($\frac{1}{1×2}$ $+….+$ $\frac{1}{8×9}$$-$$\frac{1}{9×10}$$)$ $B=-$$($$\frac{1}{1}$$-$$\frac{1}{2}$$+…+$$\frac{1}{8}$$-$$\frac{1}{9}$$-$$\frac{1}{90}$$)$ $B=-$$($$\frac{1}{1}$$-$$\frac{1}{9}$$-$$\frac{1}{90}$$)$ $B=-$$\frac{79}{90}$ Trả lời
`B = 1/(9*10) – 1/(8*9) – 1/(7*8) – … – 1/(2*3) – 1/(1*2)`
`B = – (-1/(9*10) + 1/(8*9) + 1/(7*8) + … + 1/(2*3) + 1/(1*2))`
`B = – (1/(1*2) + 1/(2*3) + … + 1/(8*9) – 1/(9*10))`
`B = – (1/1 – 1/2 + 1/2 – 1/3 + … + 1/8 – 1/9 – 1/(9*10))`
`B = – (1 – 1/9 – 1/(9*10))`
`B = – (8/9 – 1/9 + 1/10)`
`B = -79/90`
$B=$$\frac{1}{9×10}$$-$$\frac{1}{8×9}$ $-….-$ $\frac{1}{1×2}$
$B=-$$($$\frac{1}{8×9}$ $+….+$ $\frac{1}{1×2}$$-$$\frac{1}{9×10}$$)$
$B=-$($\frac{1}{1×2}$ $+….+$ $\frac{1}{8×9}$$-$$\frac{1}{9×10}$$)$
$B=-$($\frac{1}{1×2}$ $+….+$ $\frac{1}{8×9}$$-$$\frac{1}{9×10}$$)$
$B=-$$($$\frac{1}{1}$$-$$\frac{1}{2}$$+…+$$\frac{1}{8}$$-$$\frac{1}{9}$$-$$\frac{1}{90}$$)$
$B=-$$($$\frac{1}{1}$$-$$\frac{1}{9}$$-$$\frac{1}{90}$$)$
$B=-$$\frac{79}{90}$