3. Let f,g,h:Z+R be functions. Prove each of the following. a. If fO(g) and gO(h) prove that fO(h). b. If f(x)=anxn+an1xn1++a1x+a0, where an,,a0 are positive real numbers and an=0, then f(x)O(xn). c. If f(x)=1k+2k++xk, where k is a positive integer, then f(x)O(xk+1)