Consider the system of linear equations in x, y, z : x+2y+tz=0 6x+y+5tz=0 3x+t2y+f(t)z=0 where f : R R is a differentiable function. If this system has infinitely many solutions for all t R, then f
- is a constant function
- is strictly increasing on R
- is strictly decreasing on R
- has two critical points
Answer
(2) is strictly increasing on R
Solution
∵ The system is homogeneous, it has infinitely many solutions iff = 0:
= vmatrix1 & 2 & t\ 6 & 1 & 5t\ 3 & t2 & f(t) vmatrix = 0
1 (f(t) - 5t3 ) - 2 (6f(t) - 15t ) + t (6t2 - 3 ) = 0
-11f(t) + t3 + 27t = 0
f(t) = t3 + 27t11
Then f'(t) = 3t2 + 2711>0 for every t R (no critical point).
∴ f is strictly increasing on R.
Answer: (2)