✔ 最佳答案
圖片參考:
http://imgcld.yimg.com/8/n/HA00273367/o/20130220211531.jpg
(a) OP^2 = a^2 + (2a^2 - 2)^2
= 4a^4 - 7a^2 + 4
dOP/da = 16a^3 - 14a = 0 => a = 0 or a = √14/4 or -√14/4
The coordinates of P are (√14/4,-1/4)
(b) dy/dx = 4x
At P, dy/dx = √14 and the slope of OP is -1/√14
So, the slope of tangent at P * slope of OP is -1. This means that OP is the normal to the curve C at P
(c) Let |MQ| is the shortest distance from the point Q(5,-1) to the curve C
Let M(p,q)
Slope of tangent at M = 4p
Slope of MQ = (q + 1)/(p - 5)
As 4p * (q + 1)/(p - 5) = -1
4p(q + 1) = 5 - p
4pq = 5 - 5p
But q = 2p^2 - 2
Thus 4p(2p^2 - 2) = 5 - 5p
8p^3 = 5 + 3p
8p^3 - 3p - 5 = 0
(p - 1)(8p^2 - 8p + 5) = 0
p = 1 is the only real root
So, M(1,0) and MQ = √17