đ/số 10;20

(X x15 ):35 =6

X x 15        = 6 x 35

X x 15        = 210

X                =210 :15

X                =14

lên googel mà tra

Có 3 loại sách ,sách nào vậy bạn

\(A=1+5^2+5^3+...+5^{2015}+5^{2016}\)

\(5A=5+5^3+5^4+...+5^{2016}+5^{2017}\)

\(4A=\left(5+5^3+5^4+...+5^{2016}+5^{2017}\right)-\left(1+5^2+5^3+...+5^{2015}+5^{2016}\right)\)

\(=5+5^{2017}-\left(1+5^2\right)\)

\(=4+5^{2017}-5^2\)

\(A=\frac{4+5^{2017}-5^2}{4}\)

Ta có : 5A = 5 + 5^3 + 5^4 + ... + 5^2016 + 5^2017

  =>    5A - A = ( 5 + 5^3 + 5^4 + ... + 5^2016 + 5^2017 ) - ( 1 + 5^2 + 5^3 + ... + 5^2015 + 5^2016 )

  =>         4A = 4 + 5^2 + 5^2017

  =>           A = ( 4 + 5^2 + 5^2017 )/4

a: =-5/6-3/7=-35/42-18/42=-53/42

b: =2/5-4/9=18/45-20/45=-2/45

c: =-24/35

d: =2/3x-5/4=-10/12=-5/6

a: \(\Leftrightarrow x=\dfrac{1}{9}+\dfrac{5}{7}=\dfrac{52}{63}\)

b: \(\Leftrightarrow x=\dfrac{1}{10}+\dfrac{1}{15}=\dfrac{1}{6}\)

c: \(\Leftrightarrow x=\dfrac{-3}{7}-\dfrac{4}{5}+\dfrac{2}{3}=-\dfrac{59}{105}\)

d: \(\Leftrightarrow x=\dfrac{-2}{15}+\dfrac{3}{10}=\dfrac{1}{6}\)

\(A=\dfrac{31\cdot\left(31^{12}-1\right)}{31\left(31^{13}+1\right)}=\dfrac{31^{13}+1-32}{31\left(31^{13}+1\right)}=\dfrac{1}{31}-\dfrac{32}{31^{14}+31}\)

\(B=\dfrac{31\left(31^{13}-1\right)}{31\left(31^{14}+1\right)}=\dfrac{1}{31}-\dfrac{32}{31^{15}+31}\)

Dễ thấy \(31^{14}+31< 31^{15}+31\Rightarrow\dfrac{32}{31^{14}+31}>\dfrac{32}{31^{15}+31}\\ \Rightarrow\dfrac{1}{31}-\dfrac{32}{31^{14}+31}< \dfrac{1}{31}-\dfrac{32}{31^{15}+31}\)

Vậy A < B

b1 nha mn

Kiểm tra nhiêu phút z bạn:)

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