Răspuns:
Explicație pas cu pas:
a=(1+2+3+....+122+123)-2(1+2+3+.......+61)=123×124/2-2×61×62/2=
=123×62-61×62=62(123-61)=62×62=62²
a=1+2+......+52+53- 2(1+2+.....+26)=53×54/2-2×26×27/2=
=53×27-26×27=27(53-26)=27²
a=1+2+.....+86+87-2(1+2+...+43)=87×88/2-2×43×44/2=
=87×44-43×44=44(87-43)=44²
