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Bài 3:

a: a*S=a^2+a^3+...+a^2023

=>(a-1)*S=a^2023-a

=>\(S=\dfrac{a^{2023}-a}{a-1}\)

b: a*B=a^2-a^3+...-a^2023

=>(a+1)B=a-a^2023

=>\(B=\dfrac{a-a^{2023}}{a+1}\)

Sau khi thực hiện xong thì a=4 ; b=5

13 tháng 9 2015

\(a^3.a^9=a^{12};\left(a^5\right)^7=a^{35}\)

\(\left(a^6\right)^4.a^{12}=a^{24}.a^{12}=a^{36};5^6:5^4-3^3:3^2=25-3=22\)

\(4.5^2-2^5.3^0=100-32=68\)

8 tháng 5 2017

\(A=\dfrac{5}{4}+\dfrac{5}{4^2}+\dfrac{5}{4^3}+...+\dfrac{5}{4^{99}}\\ 4A=5+\dfrac{5}{4}+\dfrac{5}{4^2}+...+\dfrac{5}{4^{98}}\\ 4A-A=\left(5+\dfrac{5}{4}+\dfrac{5}{4^2}+...+\dfrac{5}{4^{98}}\right)-\left(\dfrac{5}{4}+\dfrac{5}{4^2}+\dfrac{5}{4^3}+...+\dfrac{5}{4^{99}}\right)\\ 3A=5-\dfrac{5}{4^{99}}\\ A=\left(5-\dfrac{5}{4^{99}}\right):3\\ A=\dfrac{5}{3}-\dfrac{5}{4^{99}}:3\\ A=\dfrac{5}{3}-\dfrac{5}{4^{99}\cdot3}< \dfrac{5}{3}\)

Vậy \(A< \dfrac{5}{3}\)

27 tháng 12 2017

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28 tháng 12 2017

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\(\dfrac{a+4}{a-4}=\dfrac{b+5}{b-5}\)

=>\(\left(a+4\right)\left(b-5\right)=\left(a-4\right)\left(b+5\right)\)

\(\Leftrightarrow ab-5a+4b-20=ab+5a-4b-20\)

\(\Leftrightarrow-10a=-8b\)

=>a/b=4/5

24 tháng 9 2019

Ta có: \(\frac{x}{\left(a+5\right)\left(4-a\right)}=\frac{1}{a+5}+\frac{1}{4-a}\)

\(\Leftrightarrow\frac{x}{\left(a+5\right)\left(4-a\right)}=\frac{4-a+a+5}{\left(a+5\right)\left(4-a\right)}\)

\(\Leftrightarrow\frac{x}{\left(a+5\right)\left(4-a\right)}=\frac{9}{\left(a+5\right)\left(4-a\right)}\)

\(\Leftrightarrow x=9\)

Vậy x=9