cho A=1+1/2+1/3+1/4+...+1/2400-1 chứng minh rằng 200<A<400
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`7(x-1/2)^2=9`
`(x-1/2)^2=9/7`
\(=>\left[{}\begin{matrix}x-\dfrac{1}{2}=\sqrt{\dfrac{9}{7}}\\x-\dfrac{1}{2}=-\sqrt{\dfrac{9}{7}}\end{matrix}\right.\\ < =>\left[{}\begin{matrix}x=\dfrac{3}{\sqrt{7}}+\dfrac{1}{2}\\x=-\dfrac{3}{\sqrt{7}}+\dfrac{1}{2}\end{matrix}\right.\\ < =>\left[{}\begin{matrix}x=\dfrac{6+\sqrt{7}}{2\sqrt{7}}\\x=\dfrac{-6+\sqrt{7}}{2\sqrt{7}}\end{matrix}\right.\)
7.(x-\(\dfrac{1}{2}\))2=9
7.x+\(\dfrac{1}{4}\) =9
7.x=\(\dfrac{37}{4}\)
x=\(\dfrac{37}{28}\)
\(Bài.7:\\ a,\dfrac{\left(-3\right)^{10}.15^5}{25^3.\left(-9\right)^7}=\dfrac{3^{10}.3^5.5^5}{\left(5^2\right)^3.\left(3^2\right)^6.3.\left(-3\right)}\\ =\dfrac{3^{15}.5^5}{-5^6.3^{14}}=-\dfrac{3}{5}\\ b,2^3+3.\left(\dfrac{1}{9}\right)^0-2^{-2}.4+\left[\left(-2\right)^2:\dfrac{1}{2}\right].8\\ =8+3.1-\dfrac{1}{4}.4+\left[4:\dfrac{1}{2}\right].8\\ =8+3-1+8.8\\ =11-1+64=10+64=74\)
Bài 10:
\(a,3^{35}=\left(3^7\right)^5=2187^5\\ 5^{20}=\left(5^4\right)^5=625^5\\ Vì:2187^5>625^5\left(Vì:2187>625\right)\\ \Rightarrow3^{35}>5^{20}\\ b,2^{32}=\left(2^4\right)^8=16^8\\ Vì:37^8>16^8\left(Do:37>16\right)\\ \Rightarrow37^8>2^{32}\)