Asosiy tarkibga o'tish

9.8-bo'lim : Polar koordinatali maydon

2. \(r = 7 + 3\cos \theta \) grafigining ichidagi va \(y\) o‘qining chap tomonidagi maydonni toping.

Yechim

Birinchidan, bizni qiziqtirgan mintaqa grafigining qisqacha eskizi.

A sketch of the graph of \(r = 7 + 3\cos \theta \).  It is a large oval shaped curve running from about \(x = - 4\) on the left out to \(x = 10\) on the right and reaching up and down to about \(y = \pm 7\).  The portion of the inside of the curve that lies to the left of the y-axis is shaded green.
2-qadam

Ushbu muammo uchun muammoning chegaralarini olish juda qiyin emas. Grafikning \(y\) o'qining chap tomonidagi qismini kuzatish uchun \(\frac{\pi }{2} \le \theta \le \frac{{3\pi }}{2}\) chegaralaridan foydalanishimiz kerak bo'ladi.

Esda tutingki, \(\theta \) ning kichikroq qiymatdan kattaroq qiymatga o'sishi sifatida bizni qiziqtirgan maydonni belgilovchi egri chiziq qismini kuzatish muhimdir.

3-qadam

Keyin hudud,

\[\begin{align*}A & = \int_{{\frac{\pi }{2}}}^{{\frac{{3\pi }}{2}}}{{\frac{1}{2}{{\left( {7 + 3\cos \theta } \right)}^2}\,d\theta }}\\ & = \frac{1}{2}\int_{{\frac{\pi }{2}}}^{{\frac{{3\pi }}{2}}}{{49 + 42\cos \theta + 9co{s^2}\left( \theta \right)\,d\theta }}\\ & = \frac{1}{2}\int_{{\frac{\pi }{2}}}^{{\frac{{3\pi }}{2}}}{{49 + 42\cos \theta + \frac{9}{2}\left( {1 + \cos \left( {2\theta } \right)} \right)\,d\theta }}\\ & = \frac{1}{2}\int_{{\frac{\pi }{2}}}^{{\frac{{3\pi }}{2}}}{{\frac{{107}}{2} + 42\cos \theta + \frac{9}{2}\cos \left( {2\theta } \right)\,d\theta }}\\ & = \left. {\frac{1}{2}\left( {\frac{{107}}{2}\theta + 42\sin \left( \theta \right) + \frac{9}{4}\sin \left( {2\theta } \right)} \right)} \right|_{\frac{\pi }{2}}^{\frac{{3\pi }}{2}} = \require{bbox} \bbox[2pt,border:1px solid black]{{42.0376}}\end{align*}\]

Ushbu integrallarni bajarish uchun zarur bo'lgan trig manipulyatsiyalarini bajarishingiz mumkinligiga ishonch hosil qiling. Ushbu bo'limdagi integrallarning aksariyati bunday manipulyatsiyani o'z ichiga oladi. Agar ularni qanday qilishni eslamasangiz, orqaga qayting va ko'rib chiqing Trig funksiyalarni o'z ichiga olgan integrallar bo'limi.