summation from k = 0 to n , C from 2k to 2n (-1/3)^k and
summation from k=0 to n-1 C from 2k+1 to 2n (-1/3)^k
更新1:
2^2n/3^n cosn兀/3 , 2^2n。3^1/2/3^nsinn兀/3 P.s. 。 = multiply
更新2:
I don't know the expansion of binomial.... seems like u put k = 2k into the 2nd step in 1st term , and k= 2n-k in 2nd term@@ totally confused
