s1=1+2+3+4+5+6+.....+999s1=1+2+3+4+5+6+.....+999

s1=(1+999)+(2+998)+.....+(499+501)+500s1=(1+999)+(2+998)+.....+(499+501)+500có 499 cặp

s1=1000+1000+.....+1000s1=1000+1000+.....+1000có 499 số 1000+500+500

s1=1000×499+500s1=1000×499+500

s1=499000+500s1=499000+500

s1=499500
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s2=21+23+25+....+1001s2=21+23+25+....+1001

s2=(1001−21):2+1=491s2=(1001−21):2+1=491

s2=(1001+21)×491:2s2=(1001+21)×491:2

s2=1022×491:2s2=1022×491:2

s2=501802:2s2=501802:2

s2=250901