Question

If for the curve $y = 1 + bx - x^2$, the tangent at $(1 - 2)$ is parallel to $x$-axis, then $b =$

• A. 2
• B. -2
• C. 1
• D. -1

Answer 1

Answer: (A)

2

Answer 2

$y = 1 + bx - x^2$,
$\frac{dy}{dx} = b - 2x$
Now, $\frac{dy}{dx}|_{\left(1, -2\right) } = b-2$
Since, tangent at (l, -2) Is parallel to x-axis
$\therefore \:\: \frac{dy}{dx}|_{\left(1, -2\right)} = 0$
$\Rightarrow b- 2 = 0 \Rightarrow b= 2$