a11x2+a12y2=a13

网友回答

给定任意一个二次曲面F(x,y,z)≡a11x2+a22y2+a33z2+2a12xy+2a13xz+2a23yz+2a14x+2a34z+a44=0(1)为讨论方便,我们引入如下记号.设F1(x,y,z)≡a11x+a12y+a13z+a14,F2(x,y,z)≡a12x+a22y+a23z+a24,F3(x,y,z)≡a13x+a23y+a33z+a34,F4(x,y,z)≡a14x+a24y+a34z+a44,Φ(x,y,z)=a11x2+a22y2+a33z2+2a12xy+2a13xz+2a14yz则有F(x,y,z)≡xF1(x,y,z)+yF2(x,y,z)+zF3(x,y,z)+F4(x,y,z)=0(2)定义1二次曲面(2)上满足条件F1(x0,y0,z0)=F2(x0,y0,z0)=F3(x0,y0,z0)=0的点(x0,y0,z0)叫做二次曲面(2)的奇异点,二次曲面的非奇异点叫做二次曲面的正常点

网友回答

好像是高考题答：设公差是d 那么 a3=a1+2d a5=a1+4d a7=a1+6d a10=a1+9d a13=a1+12d 代入原来的式子，就会得到 12*a1+72d=24 a1+6d=2 而s13=(a1+a13)*13/2 =(a1+a1+12d)*13/2 =2*(a1+6d)*13/2 =26