a)
11/10 + 22/10+33/10+...............99/10=11/10(1+2+3+...9) = 11/10 x 45 = 99/2
b) 111/100 + 112/100 + 113/100 + .....130/100= 1/100(111+112+113+...130)= 1/100 (130+111)30/2 = 723/20
c)ab,c+bc,a+ca,b = 1/10(abc+bca+cab)=1/10(100(a+b+c)+10(a+b+c)+a+b+c =(a+b+c)111/10
d)aa,a+bb,b+cc,c=1/10(100(a+b+c)+10(a+b+c)+a+b+c) = (a+b+c)111/10
