Question 1098559
x^2+xy+yz+zx=30 
y^2+xy+yz+zx=15
z^2+xy+yz+zx=18 
a=(xy+yz+zx)
x^2=30-a
y^2=15-a
z^2=18-a
x^2+y^2+z^2=30-a+15-a+18-a
x^2+y^2+z^2=30+15+18-3a
x^2+y^2+z^2=63-3a