a²+b²
=(a-b)²+2ab
=5²+2×3
=25+6
=31
(a+b)²
=(a-b)²+4ab
=5²+4×3
=25+12
=37
a+b=±√(a+b)²=±√37
a²-b²=(a+b)(a-b)=±√37×5=±5√37