Vì B =\(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) < 1

\(\Rightarrow\) B = \(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) <\(\dfrac{2005^{2015}+1+2004}{2005^{2016}+1+2004}\)

\(\Rightarrow\)B = \(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) < \(\dfrac{2005^{2015}+2005}{2005^{2016}+2005}\)

\(\Rightarrow\) B = \(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) < \(\dfrac{2005^{2015}+2005}{2005^{2016}+2005}\)

\(\Rightarrow\) B = \(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) < \(\dfrac{2005\left(2005^{2014}+1\right)}{2005\left(2005^{2015}+1\right)}\)

\(\Rightarrow\) B = \(\dfrac{2005^{2015}+1}{2005^{2016}+1}\) < \(\dfrac{2005^{2014}+1}{2005^{2015}+1}\) = A

Vậy A > B

C = \(\dfrac{3}{4.7}+\dfrac{3}{7.10}+.....+\dfrac{3}{37.39}\)

= \(\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+.....+\dfrac{1}{37}-\dfrac{1}{39}\)

= \(\dfrac{1}{4}-\dfrac{1}{39}\) = \(\dfrac{35}{156}\)

Vậy C = \(\dfrac{35}{156}\)

A = \(\dfrac{1}{1.2}+\dfrac{1}{2.3}+.....+\dfrac{1}{49.50}\)

= \(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+......+\dfrac{1}{49}-\dfrac{1}{50}\)

= 1 - \(\dfrac{1}{50}\) = \(\dfrac{49}{50}\)

Vậy A = \(\dfrac{49}{50}\)

B = \(\dfrac{2}{3.5}+\dfrac{2}{5.7}+.....+\dfrac{2}{37.39}\)

= \(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+....+\dfrac{1}{37}-\dfrac{1}{39}\)

= \(\dfrac{1}{3}-\dfrac{1}{39}\) = \(\dfrac{13}{39}-\dfrac{1}{39}=\dfrac{38}{39}\)

Vậy B = \(\dfrac{38}{39}\)

3 tik đâu ra

Cho hỗn hợp X gồm Fe và FeS tác dụng với dung dịch HCl dư, thu được khí Y có tỉ khối so với hidro là 9. Thành phần phần trăm theo khối lượng Fe trong hỗn hợp X là

A. 40%.

B. 50%.

C. 38,89%.

D. 61,11%.

có ngoặc ở các phép cộng nha

11 x 11 = 4 vì 1 + 1 x 1 + 1 = 4

22 x 22 = 16 vì 2 + 2 x 2 + 2 = 16

vậy 33 x 33 = 3 + 3 x 3 + 3 = 36