Consider the function. f(x)= cases a (7x-12-x2 )b |x2-7x+12 |, & x<3\\[2ex] 2 (x-3)x-[x], & x>3\\[2ex] b, & x=3, cases where [x] denotes the greatest integer less than or equal to x. If S denotes the set of all ordered pairs (a, b) such that f(x) is continuous at x=3, then the number of elements in S is :
- Infinitely many
- 4
- 2
- 1
Answer
(4) 1
Solution
For x 3-: x2-7x+12=(x-3)(x-4)>0
f(3-)= -a(x-3)(x-4)b(x-3)(x-4)=- ab
For x 3+: [x]=3 f(3+)= x 3+2 (x-3)x-3=21=2
f(3)=b
Continuity: - ab=2=b b=2, a=-4
S =\(-4,2)\: one element
Answer: (4)