1. ace
2. abcd
3. bde
4. cd
5. Yes, if k is odd. No if k is even.
(I+N)(I-N+N^2-N^3+...+N^(k-1))=N^k+I=I
6. (1/4)x^2+(1/9)y^2=1
7.(1,-1) corres. to 2^100
(1,-2) corres. to 7^100
8. 1+X1+X2+...+Xn
9. 330
10. (1,2,6)。(3,5)。(4,8)
11. 6x2^n-n-2
12. x/[(1-x)^2(1-2x)]+4/(1-2x)
13. 1/[(1-x)(1-x^2)(1-x^3)....]
14. 2^(2^(m-1))
15. (n-1)/2