4.Tính nhanh (phân số) :
A= 1/5.6 + 1/6.7 + 1/7.8 + ………. + 1/24.25
B= 2/3.5 + 3/5.8 + 11/8.19 + 13/19.32 + 25/32.57 + 30/57.87
5. Cho A =10/17 + 8/15 + 11/12 . Chứng tỏ A<2
GIÚP MIK VS MN
4.Tính nhanh (phân số) : A= 1/5.6 + 1/6.7 + 1/7.8 + ………. + 1/24.25 B= 2/3.5 + 3/5.8 + 11/8.19 + 13/19.32 + 25/32.57 + 30/57.87 5. Cho A =1
By Madelyn
Đáp án:
A = $\dfrac{1}{5.6}$ + $\dfrac{1}{6.7}$ + $\dfrac{1}{7.8}$ + …. + $\dfrac{1}{24.25}$
= $\dfrac{1}{5}$ -$\dfrac{1}{6}$ + $\dfrac{1}{6}$ -$\dfrac{1}{7}$ + $\dfrac{1}{7}$ – $\dfrac{1}{8}$ +…+ $\dfrac{1}{24}$ – $\dfrac{1}{25}$
= $\dfrac{1}{5}$ – $\dfrac{1}{25}$
= $\dfrac{5}{25}$ – $\dfrac{1}{25}$
= $\dfrac{4}{25}$
B = $\dfrac{2}{3.5}$ + $\dfrac{3}{5.8}$ + $\dfrac{11}{8.9}$ + $\dfrac{13}{19.32}$ + $\dfrac{25}{32.57}$ + $\dfrac{30}{57.87}$
= $\dfrac{1}{3}$ – $\dfrac{1}{5}$ + $\dfrac{1}{5}$ – $\dfrac{1}{8}$ +$\dfrac{1}{8}$ – $\dfrac{1}{9}$ + $\dfrac{1}{19}$ -$\dfrac{1}{32}$ + $\dfrac{1}{32}$ – $\dfrac{1}{57}$ + $\dfrac{1}{57}$ – $\dfrac{1}{87}$
= $\dfrac{1}{3}$ – $\dfrac{1}{87}$
= $\dfrac{29}{87}$ – $\dfrac{1}{87}$
= $\dfrac{28}{87}$
Bài 5 sai đề :v
Lời giải:
4.
$A=\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+…+\dfrac{1}{24.25}$
$A=(\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+…+\dfrac{1}{24}-\dfrac{1}{25})$
$A=(\dfrac{1}{5}-\dfrac{1}{25})$
$A=\dfrac{4}{25}$
$A=\dfrac{4}{25}$
;
;
;
`B= 2/3.5 + 3/5.8 + 11/8.19 + 13/19.32 + 25/32.57 + 30/57.87`
`B=(1/3-1/5)+(1/5-1/8)+(1/8-1/9)+…+(1/32-1/57)+(1/57-1/87)`
`B=1/3-1/87`
`B=28/87`
5.
`A =10/17 + 8/15 + 11/12`
`A=286/255 + 11/12`
`A=693/340`
Ta có:` A≈2`
`=>` Vô lí