The function, f(x)=(3x−7)x2/3,x∈R, is increasing for all x lying in:
(A)(−∞,1514)
(B)(−∞,0)∪(73,∞)
(C)(−∞,−1514)∪(0,∞)
(D)(−∞,0)∪(1514,∞)✓ Correct
Answer: (D)
Solution
Writing f(x)=3x5/3−7x2/3, its derivative is f′(x)=5x2/3−314x−1/3=3x1/315x−14. For x>0 this is positive once 15x−14>0, i.e. x>1514; for x<0 both numerator and denominator are negative, so the quotient is again positive. Hence f is increasing on (−∞,0)∪(1514,∞).Sign chart
JEE Main · 2020 · 03 Sep 2020 · Shift 1 · Single correct option