求大神指教 考研一年多了现在根本不会做

解：
1/（1 + ax^2 ) + b cosx = (1 - ax^2 + a^2 * x^4 - ...) + b [ 1 - (1/2!)* x^2 + (1/4!)*x^4 + ... )
= (1+b) - (a +b/2)x^2 + (a^2 +b/24)x^4 + .... ~ cx^4
故 (1+b) = 0 ， (a +b/2) = 0 ； (a^2 +b/24) = c
b = -1 ， a = 1/2； c = 1/4 - 1/24 = 5/24