Răspuns: [tex]\red{\boxed{\bf ~21~}}[/tex]
Explicație pas cu pas:
[tex]\bf \sqrt{21+2\cdot \big(1+2+3+...+20\big)} =[/tex]
[tex]\bf \sqrt{21+2\cdot\big[\big(1+20\big)\cdot 20:2\big]} =[/tex]
[tex]\bf \sqrt{21+2\cdot\big(21\cdot 20:2\big)} =[/tex]
[tex]\bf \sqrt{21+2\cdot\big(21\cdot 10\big)} =[/tex]
[tex]\bf \sqrt{21+2\cdot210} =[/tex]
[tex]\bf \sqrt{21+421} = \sqrt{441} = \sqrt{21^{2} } =\red{\boxed{\bf ~21~}}[/tex]
