4. (3 points) For integer n, consider series a(n)=i=1ni21. a. Write R code to calculate a(100) a(100)=i=1100i21 b. For n=10,20,,100, report a(n). c. Write a function. The input is x=c(10,20,30,40,50,60,70,80,90,100), the output is y=c(a(10),a(20),,a(100)). d. Plot the function in part c