Say when you're at the front of C prime, your thumb points to the right, your index points up because that's where the surface is, and then your middle finger will point out.

你在曲线C的前端，你的拇指指向右边，你给指出来了，因为那就是这个表面所在，然后你的中指就会指出。

youdao

Say when you're at the front of C prime, your thumb points to the right, your index points up because that's where the surface is, and then your middle finger will point out.

你在曲线C的前端，你的拇指指向右边，你给指出来了，因为那就是这个表面所在，然后你的中指就会指出。

youdao