Question and Answers Forum

How many 6-digits positive integers which are formed by the digits 1 to 9 are such that each of the digits in the number appears at least twice? [For instance: 121233,122221,777777 and etc.]

$$\mathrm{How}\:\mathrm{many}\:\mathrm{6}-\mathrm{digits}\:\mathrm{positive}\:\mathrm{integers}\:\mathrm{which}\:\mathrm{are} \\ $$$$\mathrm{formed}\:\mathrm{by}\:\mathrm{the}\:\mathrm{digits}\:\mathrm{1}\:\mathrm{to}\:\mathrm{9}\:\mathrm{are}\:\mathrm{such}\:\mathrm{that}\:\mathrm{each} \\ $$$$\mathrm{of}\:\mathrm{the}\:\mathrm{digits}\:\mathrm{in}\:\mathrm{the}\:\mathrm{number}\:\mathrm{appears}\:\mathrm{at}\:\mathrm{least} \\ $$$$\mathrm{twice}?\:\left[\mathrm{For}\:\mathrm{instance}:\:\mathrm{121233},\mathrm{122221},\mathrm{777777}\:\mathrm{and}\:\mathrm{etc}.\right] \\ $$

Question Number 116177 Answers: 0 Comments: 1

Question Number 116173 Answers: 0 Comments: 2

How many ways can the letters of the word DENKENMATHEMATICAL be arranged if no same letters must be together

$$\mathrm{How}\:\mathrm{many}\:\mathrm{ways}\:\mathrm{can}\:\mathrm{the}\:\mathrm{letters}\:\mathrm{of}\:\mathrm{the}\:\mathrm{word}\:\:\mathrm{DENKENMATHEMATICAL} \\ $$$$\mathrm{be}\:\mathrm{arranged}\:\mathrm{if}\:\mathrm{no}\:\mathrm{same}\:\mathrm{letters}\:\mathrm{must}\:\mathrm{be}\:\mathrm{together} \\ $$

Question Number 116167 Answers: 1 Comments: 0

Show that ∀n≥2 the equation x^n =x+n admits a unique solution u_n ∈(1,2]

$$\mathrm{Show}\:\mathrm{that}\:\forall{n}\geqslant\mathrm{2}\:\mathrm{the}\:\mathrm{equation}\:{x}^{{n}} ={x}+{n} \\ $$$$\mathrm{admits}\:\mathrm{a}\:\mathrm{unique}\:\mathrm{solution}\:\mathrm{u}_{\mathrm{n}} \in\left(\mathrm{1},\mathrm{2}\right] \\ $$

Question Number 116166 Answers: 1 Comments: 0

∀n∈N^∗ , suppose u_n =(5sin(1/n^2 )+(1/5)cos n)^n Prove that lim_(n→+∞) u_n =0

$$\forall{n}\in\mathbb{N}^{\ast} ,\:\mathrm{suppose}\:{u}_{{n}} =\left(\mathrm{5sin}\frac{\mathrm{1}}{{n}^{\mathrm{2}} }+\frac{\mathrm{1}}{\mathrm{5}}\mathrm{cos}\:{n}\right)^{{n}} \\ $$$$\mathrm{Prove}\:\mathrm{that}\:\underset{{n}\rightarrow+\infty} {\mathrm{lim}}{u}_{{n}} =\mathrm{0} \\ $$

Question Number 116162 Answers: 1 Comments: 0

Σ_(n=0) ^∞ ((2n+1)/(16^n (n^2 +3n+2))) (((2n)),(n) )^2 =(8/(3π)) m.n.july 1970.

$$\:\: \\ $$$$ \\ $$$$\:\:\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\mathrm{2}{n}+\mathrm{1}}{\mathrm{16}^{{n}} \left({n}^{\mathrm{2}} +\mathrm{3}{n}+\mathrm{2}\right)}\:\begin{pmatrix}{\mathrm{2}{n}}\\{{n}}\end{pmatrix}^{\mathrm{2}} \:=\frac{\mathrm{8}}{\mathrm{3}\pi}\: \\ $$$$ \\ $$$${m}.{n}.{july}\:\mathrm{1970}. \\ $$$$\: \\ $$

Question Number 116163 Answers: 1 Comments: 0

I need a general rule for factorising 1) a^n +b^n 2) a^n −b^n when n is even and when n is odd.

$$\mathrm{I}\:\mathrm{need}\:\mathrm{a}\:\mathrm{general}\:\mathrm{rule}\:\mathrm{for}\:\mathrm{factorising} \\ $$$$\left.\mathrm{1}\right)\:\mathrm{a}^{\mathrm{n}} +\mathrm{b}^{\mathrm{n}} \\ $$$$\left.\mathrm{2}\right)\:\mathrm{a}^{\mathrm{n}} −\mathrm{b}^{\mathrm{n}} \\ $$$$\mathrm{when}\:\mathrm{n}\:\mathrm{is}\:\mathrm{even}\:\mathrm{and}\:\mathrm{when}\:\mathrm{n}\:\mathrm{is}\:\mathrm{odd}. \\ $$

Question Number 116196 Answers: 6 Comments: 0

please solve : ∫_0 ^( (π/4)) tan^9 (x)dx =???

$$\:\:\:\:\:{please}\:{solve}\:: \\ $$$$\:\:\: \\ $$$$\:\:\:\int_{\mathrm{0}} ^{\:\frac{\pi}{\mathrm{4}}} {tan}^{\mathrm{9}} \left({x}\right){dx}\:=??? \\ $$

Question Number 116149 Answers: 0 Comments: 1

Question Number 116138 Answers: 3 Comments: 1

If α and β are the roots of the quadratic equation x^2 −10x+2=0 and α >β, find: (i) (1/β)−(1/α) (ii)α^3 −β^3

$$\mathrm{If}\:\:\alpha\:\mathrm{and}\:\beta\:\mathrm{are}\:\mathrm{the}\:\mathrm{roots}\:\mathrm{of}\: \\ $$$$\mathrm{the}\:\mathrm{quadratic}\:\mathrm{equation}\: \\ $$$$\mathrm{x}^{\mathrm{2}} −\mathrm{10x}+\mathrm{2}=\mathrm{0}\:\mathrm{and}\:\alpha\:>\beta,\:\mathrm{find}: \\ $$$$\left(\mathrm{i}\right)\:\frac{\mathrm{1}}{\beta}−\frac{\mathrm{1}}{\alpha} \\ $$$$\left(\mathrm{ii}\right)\alpha^{\mathrm{3}} −\beta^{\mathrm{3}} \\ $$

Question Number 116136 Answers: 3 Comments: 0

y′ =(y/x) +((2x^3 cos (x^2 ))/y) where y((√π)) = 0

$$\mathrm{y}'\:=\frac{\mathrm{y}}{\mathrm{x}}\:+\frac{\mathrm{2x}^{\mathrm{3}} \:\mathrm{cos}\:\left(\mathrm{x}^{\mathrm{2}} \right)}{\mathrm{y}} \\ $$$$\mathrm{where}\:\mathrm{y}\left(\sqrt{\pi}\right)\:=\:\mathrm{0} \\ $$

Question Number 116114 Answers: 1 Comments: 0

Question Number 116113 Answers: 1 Comments: 0

what′s the exponent of 12 in the expansion of 100!

$$\boldsymbol{\mathrm{what}}'\boldsymbol{\mathrm{s}}\:\boldsymbol{\mathrm{the}}\:\boldsymbol{\mathrm{exponent}}\:\boldsymbol{\mathrm{of}}\:\mathrm{12}\:\boldsymbol{\mathrm{in}}\:\boldsymbol{\mathrm{the}} \\ $$$$\boldsymbol{\mathrm{expansion}}\:\boldsymbol{\mathrm{of}}\:\mathrm{100}! \\ $$

Question Number 116112 Answers: 2 Comments: 0

show that ∫_( 0) ^( ∞) ((lnx)/(1+x^2 ))dx = 0

$$\boldsymbol{\mathrm{show}}\:\boldsymbol{\mathrm{that}}\: \\ $$$$\int_{\:\mathrm{0}} ^{\:\infty} \frac{\boldsymbol{\mathrm{lnx}}}{\mathrm{1}+\boldsymbol{\mathrm{x}}^{\mathrm{2}} }\boldsymbol{\mathrm{dx}}\:=\:\mathrm{0} \\ $$$$ \\ $$

Question Number 116226 Answers: 2 Comments: 1

(1/(2+(√2))) +(1/(3(√2)+2(√3))) +(1/(4(√3)+3(√4)))+...+(1/(100(√(99))+99(√(100))))

$$\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{2}}}\:+\frac{\mathrm{1}}{\mathrm{3}\sqrt{\mathrm{2}}+\mathrm{2}\sqrt{\mathrm{3}}}\:+\frac{\mathrm{1}}{\mathrm{4}\sqrt{\mathrm{3}}+\mathrm{3}\sqrt{\mathrm{4}}}+...+\frac{\mathrm{1}}{\mathrm{100}\sqrt{\mathrm{99}}+\mathrm{99}\sqrt{\mathrm{100}}} \\ $$

Question Number 116123 Answers: 0 Comments: 0

Study according to the values of the real α the convergence of the integral ∫_α ^(+∞) ((ln∣x∣)/( ((x(x+1)))^(1/3) ))dx

$$ \\ $$$$\mathrm{Study}\:\mathrm{according}\:\mathrm{to}\:\mathrm{the}\:\mathrm{values}\:\mathrm{of}\: \\ $$$$\mathrm{the}\:\mathrm{real}\:\alpha\:\mathrm{the}\:\mathrm{convergence}\:\mathrm{of}\:\mathrm{the} \\ $$$$\mathrm{integral}\:\:\int_{\alpha} ^{+\infty} \frac{{ln}\mid{x}\mid}{\:\sqrt[{\mathrm{3}}]{{x}\left({x}+\mathrm{1}\right)}}{dx} \\ $$

Question Number 116105 Answers: 2 Comments: 0

solve the Cauchy-Euler Differential Equation by substituting x=e^t x^3 (d^3 y/dx^3 ) + 2x^2 (d^2 y/dx^2 ) + 2y = 10x + ((10)/x)

$${solve}\:{the}\:{Cauchy}-{Euler}\: \\ $$$${Differential}\:{Equation}\:{by} \\ $$$${substituting}\:{x}={e}^{{t}} \\ $$$${x}^{\mathrm{3}} \:\frac{{d}^{\mathrm{3}} {y}}{{dx}^{\mathrm{3}} }\:+\:\mathrm{2}{x}^{\mathrm{2}} \:\frac{{d}^{\mathrm{2}} {y}}{{dx}^{\mathrm{2}} }\:+\:\mathrm{2}{y}\:=\:\mathrm{10}{x}\:+\:\frac{\mathrm{10}}{{x}} \\ $$$$ \\ $$

Question Number 116098 Answers: 0 Comments: 0

1)calculate f(x)=∫_0 ^(2π) (dθ/(x^2 −2x cosθ +1)) 0<θ<(π/2) 2)explicite ∫_0 ^(2π) ((cosθ)/((x^2 −2xcosθ +1)^2 ))dθ

$$\left.\mathrm{1}\right)\mathrm{calculate}\:\mathrm{f}\left(\mathrm{x}\right)=\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\frac{\mathrm{d}\theta}{\mathrm{x}^{\mathrm{2}} −\mathrm{2x}\:\mathrm{cos}\theta\:+\mathrm{1}}\:\:\:\:\mathrm{0}<\theta<\frac{\pi}{\mathrm{2}} \\ $$$$\left.\mathrm{2}\right)\mathrm{explicite}\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\:\frac{\mathrm{cos}\theta}{\left(\mathrm{x}^{\mathrm{2}} −\mathrm{2xcos}\theta\:+\mathrm{1}\right)^{\mathrm{2}} }\mathrm{d}\theta \\ $$

Question Number 116097 Answers: 3 Comments: 0

calculate ∫_0 ^∞ ((lnx)/(x^4 +1))dx

$$\mathrm{calculate}\:\int_{\mathrm{0}} ^{\infty} \:\frac{\mathrm{lnx}}{\mathrm{x}^{\mathrm{4}} \:+\mathrm{1}}\mathrm{dx}\: \\ $$

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