1+2+3+4+5+6+7+8+9+10 = (10×11)/2 = 110/2 =55


1+2+3+4+5+6+7+...+199+200=(200×201)/2 = (201*200) /2


20100 /2 = 10050


∑k=1 to 3 k = 1+2+3 = (3×2)/2 =3




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