Let the midpoint of MN be p.
DP=MN/2= 1
Is to find the surface area of 1/8 ball.
S=4π* 1^2* 1/8=π/2
LS self-synthesis ...
In other words, Baidu has few experts. I think it's because Baidu's problem level is low and it's tiring. To do this, it is best to develop thinking about foreign countries (such as Russia) competition.