(a+b)²+(a-b)²=11+7
a²+2ab+b²+a²-2ab+b²=18
2(a²+b²)=18
a²+b²=9
(a+b)²-(a-b)²=11-7
a²+2ab+b²-a²+2ab-b²=4
4ab=4
ab=1