f.2 MATHZ PLZ HELP VERY IMPORTANT

plz help me to to answer the question,if you do not know the answer,plz answer no~i need the steep~Thanks for your help~

1)Given that the sum of the integers from 1 to n is n(n+1)/2,ie.1+2+3+...+n=n(n+1)/2
(a)Find 1+2+3+...+50
(b)Find 1+2+3+..+100
(c)Find 51+52+53+...+100
(d)Find 21+22+23+...+50