a)
1+3+5+7+9=25 sau (a1+a5)*5/2= (1+9)*5/2= 25
b)
11+13+15+17+19=10*5+(1+2+..+9)=50+25=75
c)
111+113+115+117+119=110*5+(1+2+...+9)=550+25=575
d)
101+303+505+707+909=101 *1+101*2+101*3+101*4+101*5+104*7+101*9 =101*(1+3+5+..9)=101*10*5/2=2525
e)
11+12+13+14+15+16+17+18+19= (a1+a9)*9/2=(11+19)*9/2=30*9/2=135
f)
27+28+29+30+31+32+33= (a1+a7)*7/2 =(27+33)*7/2=60*7/2=210
g)
91+92+93+...99=(a1+a9)*9/2=( 91+99)*9/2=855
h)
236+237+238+...264= (236+264) * (264- 235)/2= 500*29/2=7250
