Question

 

S1= 1+2+3+...+999

S2=21+23+25+...+1001

S3=23+24+...+127+128

S4=15+17+19+21+...+151+153+155

 

Answer 1

\(s1=1+2+3+4+5+6+.....+999\)

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

\(s1=1000+1000+.....+1000\)có 499 số 1000\(+500\)

\(s1=1000\times499+500\)

\(s1=499000+500\)

\(s1=499500\)

Answer 2

\(s2=21+23+25+....+1001\)

\(s2=\left(1001-21\right):2+1=491\)

\(s2=\left(1001+21\right)\times491:2\)

\(s2=1022\times491:2\)

\(s2=501802:2\)

\(s2=250901\)