a = 1 + 2 + 3 + ... + 24 +13 * 25.​


Răspuns :

[tex]\it a=1+2+3+\ ...\ +24+25\cdot13=\dfrac{24\cdot25}{2}+25\cdot13=12\cdot25+25\cdot13=\\ \\ =25(12+13)=25\cdot25=625[/tex]