X1^2-x2^2=21
c=?
x^2-7x+c=0
по т. Вието
x1+x2=-p=-(-7)=7
x1*x2=c
x1=(7-x2)
(7-x2)^2-x2^2=21
49-14x2+x2^2-x2^2=21
49-14x2=21
-14x2=21-49
-14x2=-28|(-14)
x2=2
x1=7-x2=7-2
x1=5
x1^2-x2^2=21
5^2-2^2=21
25-4=21
21=21
x1+x2=7
x1*x2=c
5+2=7
5*2=10
c=10
x^2-7x+10=0