20061019, 14:19  #1 
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
1000010101011_{2} Posts 
Easy
How do you calculate (and what is) 1+2+3+4...97+98+99+100 in under 10 seconds?

20061019, 14:39  #2 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
2^{2}·3^{2}·13^{2} Posts 
Divide into 2 sets, 150 & 51100
Reverse the order of the second set Imagine the second set written below the first set Add each vertical pair (the pattern emerges), all pairs add to 101 Clearly there are 50 such pairs Answer is 101*50 This is the arithmetic progression Last fiddled with by retina on 20061019 at 14:45 Reason: spoiler tags added 
20061019, 16:22  #3  
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
17·251 Posts 
Quote:


20061019, 17:14  #4 
Bronze Medalist
Jan 2004
Mumbai,India
2^{2}·3^{3}·19 Posts 
Arithmetic progression.
Sorry to correct you but Retina is right, tho' you are also right tho' your method is not orthodox. This problem was solved in a jiffy by Gauss, when he was 6 yrs. old , one of the three greatest mathematicians, who ever lived,[ Archimedes, Newton and Gauss] I'm glad you mentioned it as it has refreshed my aging memory. Mally Last fiddled with by mfgoode on 20061019 at 17:16 Reason: Add on 
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