Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Applications of Derivatives

Question:

If y = f(x) be the equation of the line touching the line y = 2x + 3 at x = 2, then

Options:

$f'(2)=3$

$2 f(2)=7 f'(2)$

$f(2)+f'(2)+f''(2)=2$

none of these

Correct Answer:

$2 f(2)=7 f'(2)$

Explanation:

Since $y=2 x+3$ touches the curve y = f(x) at x = 2.

∴  $\left(\frac{d y}{d x}\right)_{x=2}$ = (Slope of the line y = 2x + 3)

⇒  f'(2) = 2

Also, $x=2 \Rightarrow y=2 \times 2+3=7$

Clearly, point (2, 7) lies on the curve y = f(x).

∴  7 = f(2)

Hence, $7 f'(2)=2 f(2)$.