TK bài làm nhé:

Ta có: \(1^2+3^2+5^2+\cdots+99^2\)

\(=1^2+2^2+\cdots+100^2-\left(2^2+4^2+\cdots+100^2\right)\)

\(=\left(1^2+2^2+\cdots+100^2\right)-2^2\left(1^2+2^2+\cdots+50^2\right)\)

\(=\frac{100\left(100+1\right)\left(2\cdot100+1\right)}{6}-4\cdot\frac{50\cdot\left(50+1\right)\left(2\cdot50+1\right)}{6}\)

\(=\frac{100\cdot101\cdot201}{6}-\frac{2\cdot50\cdot51\cdot101}{3}=50\cdot101\cdot67-100\cdot17\cdot101\)

\(=101\cdot50\left(67-2\cdot17\right)=5050\cdot\left(67-34\right)=33\cdot5050\)

=166650

A. 1155 nha bạn 

bạn so sánh từng số hạng là được

hello các bạn mình đang có tâm trạng rất buồn😔😔😔😔😔

uses crt;

var i,n,s:longint;

begin

clrscr;

readln(n);

s:=0;

for i:=1 to n do 

  s:=s+sqr((2*i-1));

writeln(s);

readln;

end.

= 50

Nho t ick nha

     \(\left(102+82+62+42+22\right)-\left(12+32+53+72+92\right)\)

\(=102+82+62+42+22-12-32-52-72-92\)

\(=\left(102-92\right)+\left(82-72\right)+\left(62-52\right)+\left(42-32\right)+\left(22-12\right)\)

\(=10+10+10+10+10\)

\(=10.5\)

\(=50\)

\(S=1^2+3^2+5^2+\cdots+199^2\)

\(=\left(1^2+2^2+3^2+4^2+\cdots+200^2\right)-\left(2^2+4^2+\cdots+200^2\right)\)

\(=\left(1^2+2^2+\cdots+200^2\right)-2^2\left(1^2+2^2+\cdots+100^2\right)\)

\(=\frac{200\left(200+1\right)\left(2\cdot200+1\right)}{6}-4\cdot\frac{100\left(100+1\right)\left(2\cdot100+1\right)}{6}\)

\(=\frac{200\cdot201\cdot401}{6}-\frac23\cdot100\cdot101\cdot201\)

\(=100\cdot67\cdot401-100\cdot101\cdot134\)

\(=100\cdot67\left(401-101\cdot2\right)=6700\cdot199=1333300\)

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