Optimization or extremal value of an expression via completing the square
The question asks for the minimum or maximum possible value of a variable or expression, solvable by rewriting a quadratic relationship (e.g., completing the square or using discriminant conditions).
If the sum of the square of the roots of the equation $x^{2} - (\sin\alpha - 2)x - (1+\sin\alpha) = 0$ is least, then $\alpha$ is equal to (1) $\frac{\pi}{6}$ (2) $\frac{\pi}{4}$ (3) $\frac{\pi}{3}$ (4) $\frac{\pi}{2}$
If $a \in R$ and the equation $- 3 ( x - [ x ] ) ^ { 2 } + 2 ( x - [ x ] ) + a ^ { 2 } = 0$ (where $[ x ]$ denotes the greatest integer $\leq x )$ has no integral solution, then all possible values of $a$ lie in the interval (1) $( - 2 , - 1 )$ (2) $( - \infty , - 2 ) \cup ( 2 , \infty )$ (3) $( - 1,0 ) \cup ( 0,1 )$ (4) $( 1,2 )$
If $\lambda \in R$ is such that the sum of the cubes of the roots of the equation $x ^ { 2 } + ( 2 - \lambda ) x + ( 10 - \lambda ) = 0$ is minimum, then the magnitude of the difference of the roots of this equation is : (1) $4 \sqrt { 2 }$ (2) 20 (3) $2 \sqrt { 5 }$ (4) $2 \sqrt { 7 }$
If $m$ is chosen in the quadratic equation $\left( m ^ { 2 } + 1 \right) x ^ { 2 } - 3 x + \left( m ^ { 2 } + 1 \right) ^ { 2 } = 0$ such that the sum of its roots is greatest, then the absolute difference of the cubes of its roots is: (1) $4 \sqrt { 3 }$ (2) $10 \sqrt { 5 }$ (3) $8 \sqrt { 3 }$ (4) $8 \sqrt { 5 }$
Let $q$ be the maximum integral value of $p$ in $[0, 10]$ for which the roots of the equation $x^{2} - px + \frac{5}{4}p = 0$ are rational. Then the area of the region $\left\{(x, y): 0 \leq y \leq (x - q)^{2},\, 0 \leq x \leq q\right\}$ is (1) 243 (2) 25 (3) $\frac{125}{3}$ (4) 164
Consider the following quadratic equations in $x$ $$x^2+2x-15=0 \tag{1}$$ $$2x^2+3x+a^2+12a=0 \tag{2}$$ Let us denote the two solutions of (1) by $\alpha$ and $\beta$ ($\alpha < \beta$). We are to find the range of values which $a$ in (2) can take, in order that (2) has two real solutions $\gamma$ and $\delta$ and they satisfy $$\alpha < \gamma < \beta < \delta.$$ (1) $\alpha = \mathbf{AB}$ and $\beta = \mathbf{C}$. (2) When we set $b = a^2+12a$, from the condition $\alpha < \gamma$ we have $$b > \mathbf{DEF}$$ and from the condition $\gamma < \beta < \delta$ we have $$b < \mathbf{GHI}.$$ Hence the range of the values which $a$ can take is $$\mathbf{JK} < a < \mathbf{LM}, \quad \mathbf{NO} < a < \mathbf{PQ},$$ where $\mathrm{JK} < \mathrm{NO}$.