UPSC NDA I 2021 Mathematics Solved Paper

Concept:
The general form of quadratic equation: ax2+bx+c=0
The discriminant of the equation is D=b2−4ac - D>0, roots are unequal and real - D<0, roots are imaginary - D=0, roots are real and equal
Calculation:
Given : x2+2x+k=0
On comparing with general form
a=1,b=2,c=k
Roots are real
D≥0
b2−4ac≥0
22−4×1×k≥0
4−4k≥0
4≥4k
1≥k
k≤1