（a+b)^2=a^2+2ab+b^2=(a-b)^2+4ab=25+16=41所以a+b=+或-根号下41。3（a+b）=3x41^1/2ps:a^2为a的平方。a^1/2为 根号下a.
(a+b)^2=(a-b)^2+4ab=5^2+4*4=41,所以，(a+b)=±√41，3a+3b=3(a+b)=±3√41.
