Question

1+3 =2*2
1+3+5. =3*3
1+3+5+7=4*4
1+3+5+7+9=5*5
By observing the above pattern find 21+23++25+...+51

Answer 1

21+23+25+...+51 = (1+3+5+...+51) - (1+3+5+...+19)
= 26^2 - 10^2
= 676-100
= 576