It seems to be strange to you there is a blind spot(点) on the eyes. Here is an interesting experiment that can make something disappear, when one eye is open.
Make a card about the size of a postcard and write two English letters L and R on it, L on the left and R on the right. First, hold the card about 80 cm away and you see both the letters. Then close your right eye and look at the letter R only with your left eye. And now, as you move the card slowly towards you, you'll find the letter L disappearing. But if you move the card nearer to your face, the letter will be seen again. Now do the same experiment with your left eye closed, you'll find the letter R disappearing.
Why does the letter disappear? It is because there is a blind spot on the eye. When the image of the letter falls on the blind spot, it won't be seen. That is why either of the letters disappears.
在点(3,2)处的切线与直线 
垂直，则a等于 （ ）