rï¼a(1+cosθ),r'ï¼-asinθ
å©ç¨å¯¹ç§°æ§
é¿åº¦=2â«(0,Ï)âr^2+r'^2dθ
=2â«(0,Ï)âa^2(2+2cosθ)dθ
=2aâ«(0,Ï)â4cos^2(θ/2)dθ
=4aâ«(0,Ï)cos(θ/2)dθ
=8aâ«(0,Ï)cos(θ/2)dθ/2
=8asin(θ/2)|(0,Ï)
=8a
é¢ç§¯=2*1/2â«(0,Ï)r^2dθ
=â«(0,Ï)a^2(1+cosθ)^2dθ
=4a^2â«(0,Ï)cos^4(θ/2)dθ
=8a^2â«(0,Ï)cos^4(θ/2)dθ/2 (令θ/2=t)
=8a^2â«(0,Ï/2)cos^4tdt
=8a^2*3/4*1/2*Ï/2
=3/2*Ïa^2
追é®çæ¡æ¯æ£ç¡®çï¼
请é®ä¸ºä»ä¹é¿åº¦=2â«(0,Ï)âr^2+r'^2dθï¼
è¿ä¸æ¯ædsåædθåï¼åæ¥çå½æ°æ¯1ï¼æ¯råθçå½æ°ï¼å为θçå½æ°ä¸æ¯åºè¯¥ä¹ä»¥
â[(dr/dθ)^2+(dθ/dθ)^2]
ï¼â[(dr/dθ)^2+1]
åï¼ä¸ºä»ä¹æ¯âr^2+r'^2ï¼
追ç弧微å:ds=âr^2+r'^2dθ
ä»»ä½ç§¯åæç»é½è¦å为å®ç§¯åæ¥è®¡ç®ç.
追é®3ks