CH+2C Ch,+Ch X E(C,Ch)= S+2C C s bab +C bbb Y aE OE E取极值的条件 b aE 1 aX X aY acY ac a aE 1 aX X aY 2 0 ac. Y ac. Ya OX E OC oX aY EY X c S c c S c S c H c c H c H E c c a a a a b a b b b b a a a a b a b b b b a b = + + + + = 2 2 2 2 2 2 ( , ) E取极值的条件 : 0, = 0   =   a b c c E E 即：        =   −   =   =   −   =   0 c Y Y X c X Y 1 c E 0 c Y Y X c X Y 1 c E b 2 b b a 2 a a        =   −   =   −   0 0 b b a a c Y E c X c Y E c X