The quadratic equation with rational coefficients whose one root is 1+2 is
x2+2x+1= 0
x2−2x−1=0
x2+2x−1=0
(x−1)2=0
One root is :α=1+2 β=1−2
quadratic equation is x2-α+βx+αβ=0
Required quadratic equation is x2−2x−1=0