a的平方+b的平方：把（a-b）平方得：a²-2ab+b²=36 然后a²-2ab+b²+2ab=a²+b²=36+14=50
a+b：(a+b)^2=(a-b)^2+4ab
(a+b)^2=6^2+4*7
(a+b)^2=36+28
(a+b)^2=64
a+b=±8