Related Formula
For ax²+bx+c=0 with a > 0 , if a point k lies strictly between the roots, f(k) < 0.$$\text{For } ax^2+bx+c=0 \text{ with } a > 0 \text{, if a point } k \text{ lies strictly between the roots, } f(k) < 0.$$
Core Logic
Given f(x) = x² + 2ax + (3a + 10)$f(x) = x^2 + 2ax + (3a + 10)$.
Since the coefficient of x²$x^2$ is 1 > 0$1 > 0$, the parabola opens upward.
For 1$1$ to lie between the roots α$\alpha$ and β$\beta$, the value of the function at x = 1$x = 1$ must be strictly less than 0$0$.
f(1) < 0$f(1) < 0$
Step 1: Evaluate Inequality
f(1) = 1² + 2a(1) + 3a + 10 < 0$$f(1) = 1^2 + 2a(1) + 3a + 10 < 0$$
1 + 2a + 3a + 10 < 0$$1 + 2a + 3a + 10 < 0$$
5a + 11 < 0$5a + 11 < 0$
a < -(11)/(5)$$a < -\frac{11}{5}$$
So, a in (-∞, -(11)/(5))$a \in \left(-\infty, -\frac{11}{5}\right)$.
Pattern Recognition
When a specified value k$k$ lies between roots, a · f(k) < 0$a \cdot f(k) < 0$. If a>0$a>0$, this simplifies to f(k) < 0$f(k) < 0$. No need to check discriminant Δ > 0$\Delta > 0$ manually because a f(k) < 0$a f(k) < 0$ guarantees real distinct roots.
Chapter Mix
Class 11 Maths: Quadratic Equations