高手在此 講笑
1-1/2+...+(-1/2)^(n-1)=2/3[1-(-1/2)^n]
ASSUME 1-1/2+...+(-1/2)^(k-1)=2/3[1-(-1/2)^k]
n=k+1
LHS:
1-1/2+...+(-1/2)^(k-1)+(-1/2)^k
=2/3[1-(-1/2)^k]+(1/2)^k 1-1/2+...+(-1/2)^(k-1)=2/3[1-(-1/2)^k]
=2/3-(2/3)(-1/2)^k+(-1/2)^k
=2/3+(1/3)(-1/2)^k
RHS:
2/3[1-(-1/2)^(k+1)]
=2/3-(2/3)(-1/2)^(k+1)
=2/3-(2/3)[(-1/2)^k](-1/2)
=2/3+(1/3)(-1/2)^k
=LHS
[ 本帖最後由 Believe. 於 2011-10-19 18:28 編輯 ]