A matrix is singular when its determinant equals zero.
For the matrix −1a11b2011, the determinant must equal zero.
Expanding along Row 1 (which contains a zero):
det(A)=(−1)×b211−(1)×a111+(0)×a1b2
The 2×2 determinants:
b211=(b)(1)−(1)(2)
=b−2
a111=(a)(1)−(1)(1)
=a−1
det(A)=(−1)(b−2)−(1)(a−1)+0
=−b+2−a+1
=−a−b+3
Since the matrix is singular:
det(A)=0
−a−b+3=0
−a−b=−3
a+b=3
Therefore, the relation between a and b is a+b=3.