x1+x2=13
x1*x2=5
x1²+2x1x2+x2²=169
2x1*x2=10
x1²+x2²=169-10= 159
√8-2√7- √8+2√7 =√(1-√7)²-√(1+√7)²=√7-1-(1+√7)=√7-1-1-√7=-2