Question Number 37317 by rahul 19 last updated on 11/Jun/18
$$\int\:\frac{\mathrm{acos}\:{x}+{b}}{\left({a}+{b}\mathrm{cos}\:{x}\right)^{\mathrm{2}} }{dx}\:=\:? \\ $$
Commented by math khazana by abdo last updated on 12/Jun/18
$${changement}\:{tan}\left(\frac{{x}}{\mathrm{2}}\right)\:=\:{t}\:{give}\: \\ $$$${I}\:=\:\int\:\:\frac{{a}\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{1}+{t}^{\mathrm{2}} }\:+{b}}{\left({a}\:+{b}\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{1}+{t}^{\mathrm{2}} }\right)^{\mathrm{2}} }\:\:\frac{\mathrm{2}{dt}}{\mathrm{1}+{t}^{\mathrm{2}} } \\ $$$$=\mathrm{2}\:\int\:\:\:\frac{{a}\left(\mathrm{1}−{t}^{\mathrm{2}} \right)\:\:+{b}\:+{bt}^{\mathrm{2}} }{\left(\mathrm{1}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} \frac{\left({a}+{at}^{\mathrm{2}} \:+{b}−{bt}^{\mathrm{2}} \right)^{\mathrm{2}} }{\left(\mathrm{1}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }}\:{dt} \\ $$$$=\:\mathrm{2}\:\int\:\:\:\:\:\:\frac{{a}+{b}\:+\left({b}−{a}\right){t}^{\mathrm{2}} }{\left\{{a}+{b}\:\:+\left({a}−{b}\right){t}^{\mathrm{2}} \right\}^{\mathrm{2}} }\:{dt} \\ $$$${case}\:\mathrm{1}\:\:{a}>{b}\:\: \\ $$$${I}\:\:=\mathrm{2}\:\:\int\:\:\frac{\left({a}+{b}\right)\left\{\:\mathrm{1}−\left({a}−{b}\right){t}^{\mathrm{2}} \right\}}{\left({a}+{b}\right)^{\mathrm{2}} \left\{\:\:\mathrm{1}+\frac{{a}−{b}}{{a}+{b}}{t}^{\mathrm{2}} \right\}^{\mathrm{2}} }\:{dt} \\ $$$$=\frac{\mathrm{2}}{{a}+{b}}\:\int\:\:\:\:\:\frac{\mathrm{1}−\frac{{a}−{b}}{{a}+{b}}{t}^{\mathrm{2}} }{\left\{\mathrm{1}\:+\frac{{a}−{b}}{{a}+{b}}{t}^{\mathrm{2}} \right\}^{\mathrm{2}} }{dt}\:\:{changement} \\ $$$$\sqrt{\frac{{a}−{b}}{{a}+{b}}}{t}\:={x}\:{give} \\ $$$${I}\:\:=\:\frac{\mathrm{2}}{{a}+{b}}\:\int\:\:\:\frac{\mathrm{1}−{x}^{\mathrm{2}} }{\left\{\mathrm{1}+{x}^{\mathrm{2}} \right\}^{\mathrm{2}} }\:\sqrt{\frac{{a}+{b}}{{a}−{b}}}\:{dx} \\ $$$$=\:\frac{\mathrm{2}}{\:\sqrt{{a}^{\mathrm{2}} \:−{b}^{\mathrm{2}} }}\:\int\:\:\:\:\frac{\mathrm{1}−{x}^{\mathrm{2}} }{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx}\:\:{but} \\ $$$$\int\:\:\frac{\mathrm{1}−{x}^{\mathrm{2}} }{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }{dx}\:=\:\int\:\:\frac{\mathrm{1}+{x}^{\mathrm{2}} \:−\mathrm{2}{x}^{\mathrm{2}} }{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }{dx} \\ $$$$=\:\int\:\:\:\:\frac{{dx}}{\mathrm{1}+{x}^{\mathrm{2}} }\:\:\:−\int\:\:\:\frac{\mathrm{2}{x}^{\mathrm{2}} }{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }{dx} \\ $$$$={arctanx}\:\:−\int\:\:\:\frac{\mathrm{2}{x}^{\mathrm{2}} }{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }{dx}\:\:{by}\:{parts} \\ $$$${u}^{'} \:\:\:\:=\:\frac{−\mathrm{2}{x}}{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{and}\:\:{u}\:={x} \\ $$$$\int\:\:\:\:\frac{−\mathrm{2}{x}}{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{x}\:{dx}\:=\:\frac{\mathrm{1}}{\mathrm{1}+{x}^{\mathrm{2}} }\:{x}\:\:−\int\:\:\frac{\mathrm{1}}{\mathrm{1}+{x}^{\mathrm{2}} }\:{dx} \\ $$$$=\frac{{x}}{\mathrm{1}+{x}^{\mathrm{2}} }\:\:−{arctan}\left({x}\right)\:\Rightarrow\: \\ $$$$\:{I}\:\:=\:\frac{\mathrm{2}}{\:\sqrt{{a}^{\mathrm{2}} \:−{b}^{\mathrm{2}} }}\left\{\:\:{arctan}\left({x}\right)\:\:+\frac{{x}}{\mathrm{1}+{x}^{\mathrm{2}} }\:−{arctan}\left({x}\right)\right\}\:+{c}\: \\ $$$${I}\:=\:\:\frac{\mathrm{2}{x}}{\:\sqrt{{a}^{\mathrm{2}} \:−{b}^{\mathrm{2}} }\left(\mathrm{1}+{x}^{\mathrm{2}} \right)} \\ $$
Commented by math khazana by abdo last updated on 12/Jun/18
$${case}\:\mathrm{2}\:\:{a}<{b}\:\:{we}\:{get}\: \\ $$$${I}\:=\:\frac{\mathrm{2}}{{a}+{b}}\:\int\:\:\frac{\mathrm{1}\:+\frac{{b}−{a}}{{b}+{a}}{t}^{\mathrm{2}} }{\left\{\mathrm{1}−\frac{{b}−{a}}{{b}+{a}}{t}^{\mathrm{2}} \right\}^{\mathrm{2}} }\:{dt}\:\:{and}\:{changement} \\ $$$$\sqrt{\frac{{b}−{a}}{{b}+{a}}}{t}\:={x}\:{give}\: \\ $$$${I}\:=\:\frac{\mathrm{2}}{{a}+{b}}\:\sqrt{\frac{{b}+{a}}{{b}−{a}}}\:\int\:\:\:\frac{\mathrm{1}+{x}^{\mathrm{2}} }{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }{dx} \\ $$$$=\:\frac{\mathrm{2}}{\:\sqrt{{b}^{\mathrm{2}} \:−{a}^{\mathrm{2}} }}\:\int\:\:\:\:\:\frac{\mathrm{1}+{x}^{\mathrm{2}} }{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx}\:\:{but} \\ $$$$\int\:\:\:\:\:\frac{\mathrm{1}+{x}^{\mathrm{2}} }{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx}\:=\:\int\:\frac{\mathrm{1}−{x}^{\mathrm{2}} \:+\mathrm{2}{x}^{\mathrm{2}} }{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx} \\ $$$$=\:\int\:\:\:\:\frac{{dx}}{\mathrm{1}−{x}^{\mathrm{2}} }\:\:\:+\:\:\int\:\:\:\frac{\mathrm{2}{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx} \\ $$$$={ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\:\:+\:\int\:\:\frac{\mathrm{2}{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx}\:\:{by}\:{parts} \\ $$$${u}^{'} \:\:=\:\frac{\mathrm{2}{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:\:{and}\:{v}\:={x}\:\Rightarrow \\ $$$$\int\:\:\:\:\frac{\mathrm{2}{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} }\:{dx}\:=\:\:\frac{\mathrm{1}}{\mathrm{1}−{x}^{\mathrm{2}} }\:{x}\:−\:\int\:\:\frac{\mathrm{1}}{\mathrm{1}−{x}^{\mathrm{2}} }\:{dx} \\ $$$$=\:\frac{{x}}{\mathrm{1}−{x}^{\mathrm{2}} }\:\:\:−{ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\:\Rightarrow \\ $$$${I}\:=\:\:\frac{\mathrm{2}}{\:\sqrt{{b}^{\mathrm{2}} \:−{a}^{\mathrm{2}} }}\left\{\:\:{ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\:\:+\frac{{x}}{\mathrm{1}−{x}^{\mathrm{2}} }\:−{ln}\mid\frac{\mathrm{1}+{x}}{\mathrm{1}−{x}}\mid\right. \\ $$$${I}\:=\:\frac{\mathrm{2}{x}}{\:\sqrt{{b}^{\mathrm{2}} \:−{a}^{\mathrm{2}} }\left(\mathrm{1}−{x}^{\mathrm{2}} \right)}\:\:+{c} \\ $$
Commented by math khazana by abdo last updated on 12/Jun/18
$${I}\:=\:\frac{\mathrm{2}{x}}{\:\sqrt{{a}^{\mathrm{2}} −{b}^{\mathrm{2}} }\left(\mathrm{1}+{x}^{\mathrm{2}} \right)}\:\:+{c}\: \\ $$
Commented by math khazana by abdo last updated on 12/Jun/18
$${x}\:=\sqrt{\frac{{a}−{b}}{{a}+{b}}}\:{tan}\left(\frac{{x}}{\mathrm{2}}\right)\:{at}\:{case}\:\mathrm{1}\:{and}\:{x}\:=\sqrt{\frac{{b}−{a}}{{b}+{a}}}\:{tan}\left(\frac{{x}}{\mathrm{2}}\right) \\ $$$${at}\:{case}\:\mathrm{2}\:. \\ $$
Answered by behi83417@gmail.com last updated on 12/Jun/18
$${tg}\frac{{x}}{\mathrm{2}}={t}\Rightarrow\left(\mathrm{1}+{t}^{\mathrm{2}} \right){dx}=\mathrm{2}{dt},{cosx}=\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{1}+{t}^{\mathrm{2}} } \\ $$$$\Rightarrow{I}=\int\frac{{a}\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{1}+{t}^{\mathrm{2}} }+{b}}{\left({a}+{b}\frac{\mathrm{1}−{t}^{\mathrm{2}} }{\mathrm{1}+{t}^{\mathrm{2}} }\right)^{\mathrm{2}} }.\frac{\mathrm{2}{dt}}{\mathrm{1}+{t}^{\mathrm{2}} }= \\ $$$$=\int\frac{{a}+{b}−\left({a}−{b}\right){t}^{\mathrm{2}} }{\left({a}+{b}+\left({a}−{b}\right){t}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}=\int\frac{{m}−{nt}^{\mathrm{2}} }{\left({m}+{nt}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}= \\ $$$$=\frac{{m}}{{n}^{\mathrm{2}} }\int\frac{{dt}}{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }−\frac{\mathrm{1}}{{n}}\int\frac{{t}^{\mathrm{2}} }{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}={AI}_{\mathrm{1}} +{BI}_{\mathrm{2}} \\ $$$$\left.\mathrm{1}\right){I}_{\mathrm{1}} =\int\frac{{dt}}{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }=\frac{{n}}{{m}}\int\frac{\frac{{m}}{{n}}+{t}^{\mathrm{2}} −{t}^{\mathrm{2}} }{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}= \\ $$$$=\frac{{n}}{{m}}\left[\int\frac{{dt}}{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)}−\int\frac{{t}^{\mathrm{2}} }{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}\right]= \\ $$$$=\frac{{n}}{{m}}\left[\sqrt{\frac{{n}}{{m}}}.{tg}^{−\mathrm{1}} \left(\frac{{t}}{\:\sqrt{\frac{{m}}{{n}}}}\right)+{nI}_{\mathrm{2}} \right] \\ $$$${I}_{\mathrm{2}} =\int\frac{{t}^{\mathrm{2}} }{\left(\frac{{m}}{{n}}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }{dt}=−\frac{{t}}{\mathrm{2}\left({t}^{\mathrm{2}} +\frac{{m}}{{n}}\right)}+\frac{{tg}^{−\mathrm{1}} \frac{{t}}{\:\sqrt{\frac{{m}}{{n}}}}}{\mathrm{2}\sqrt{\frac{{m}}{{n}}}} \\ $$$$\Rightarrow{I}=\frac{{m}}{{n}^{\mathrm{2}} }.{I}_{\mathrm{1}} −\frac{\mathrm{1}}{{n}}.{I}_{\mathrm{2}} =\frac{{m}}{{n}^{\mathrm{2}} }.\frac{{n}}{{m}}\left(\sqrt{\frac{{n}}{{m}}}.{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}.{t}\right)−\right. \\ $$$$\left.−\frac{{nt}}{\mathrm{2}\left({t}^{\mathrm{2}} +\frac{{m}}{{n}}\right)}+\frac{\mathrm{1}}{\mathrm{2}}{n}\sqrt{\frac{{n}}{{m}}}{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)\right)− \\ $$$$−\frac{\mathrm{1}}{{n}}\left(−\frac{{t}}{\mathrm{2}\left({t}^{\mathrm{2}} +\frac{{m}}{{n}}\right)}+\frac{\mathrm{1}}{\mathrm{2}}\sqrt{\frac{{n}}{{m}}}.{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)\right)= \\ $$$$=\frac{\mathrm{1}}{\:\sqrt{{mn}}}{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)−\frac{{t}}{\mathrm{2}\left({t}^{\mathrm{2}} +\frac{{m}}{{n}}\right)}+\frac{\mathrm{1}}{\mathrm{2}}\sqrt{\frac{{n}}{{m}}}{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)− \\ $$$$−\frac{{t}}{\mathrm{2}{n}\left({t}^{\mathrm{2}} +\frac{{m}}{{n}}\right)}+\frac{\mathrm{1}}{\mathrm{2}\sqrt{{mn}}}{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)= \\ $$$$=\frac{\mathrm{3}+{n}}{\mathrm{2}\sqrt{{mn}}}{tg}^{−\mathrm{1}} \left(\sqrt{\frac{{n}}{{m}}}{t}\right)−\frac{{n}+\mathrm{1}}{{n}}.\frac{{t}}{{t}^{\mathrm{2}} +\frac{{m}}{{n}}}+{C}= \\ $$$$=\frac{\mathrm{3}+\boldsymbol{{a}}−\boldsymbol{{b}}}{\mathrm{2}\sqrt{\boldsymbol{{a}}^{\mathrm{2}} −\boldsymbol{{b}}^{\mathrm{2}} }}.\boldsymbol{{tg}}^{−\mathrm{1}} \left(\sqrt{\frac{\boldsymbol{{a}}−\boldsymbol{{b}}}{\boldsymbol{{a}}+\boldsymbol{{b}}}}.\boldsymbol{{tg}}\frac{\boldsymbol{{x}}}{\mathrm{2}}\right)− \\ $$$$−\left(\boldsymbol{{a}}−\boldsymbol{{b}}+\mathrm{1}\right).\frac{\boldsymbol{{tg}}\frac{\boldsymbol{{x}}}{\mathrm{2}}}{\left(\boldsymbol{{a}}−\boldsymbol{{b}}\right)\boldsymbol{{tg}}^{\mathrm{2}} \frac{\boldsymbol{{x}}}{\mathrm{2}}+\left(\boldsymbol{{a}}+\boldsymbol{{b}}\right)}\:.\blacksquare \\ $$
Answered by rahul 19 last updated on 13/Jun/18
$$\mathrm{Guys}\:\mathrm{i}\:\mathrm{gave}\:\mathrm{this}\:\mathrm{problem}\:\mathrm{just}\:\mathrm{to}\:\mathrm{realise} \\ $$$$\mathrm{that}\:\mathrm{a}\:\mathrm{short}\:\mathrm{trick}\:\mathrm{exists}\:\mathrm{in}\:\mathrm{this}\:\mathrm{form}. \\ $$$$\mathrm{Just}\:\mathrm{multiplying}\:\mathrm{and}\:\mathrm{dividing}\:\mathrm{by} \\ $$$$\mathrm{cosec}^{\mathrm{2}} {x}\:{will}\:{suffice}. \\ $$