JEE MAIN - Mathematics (2021 - 27th July Evening Shift - No. 11)
Let $$\alpha = \mathop {\max }\limits_{x \in R} \{ {8^{2\sin 3x}}{.4^{4\cos 3x}}\} $$ and $$\beta = \mathop {\min }\limits_{x \in R} \{ {8^{2\sin 3x}}{.4^{4\cos 3x}}\} $$. If $$8{x^2} + bx + c = 0$$ is a quadratic equation whose roots are $$\alpha$$1/5 and $$\beta$$1/5, then the value of c $$-$$ b is equal to :
42
47
43
50
Explanation
$$\alpha = \mathop {\max }\limits_{x \in R} \{ {8^{2\sin 3x}}{.4^{4\cos 3x}}\} $$
$$ = \max \{ {2^{6\sin 3x}}{.2^{8\cos 3x}}\} $$
$$ = max\{ {2^{6\sin 3x + 8\cos 3x}}\} $$
and $$\beta = \min \{ {8^{2\sin 3x}}{.4^{4\cos 3x}}\} = \min \{ {2^{6\sin 3x + 8\cos 3x}}\} $$
Now range of $$6sin3x + 8cos3x$$
$$ = \left[ { - \sqrt {{6^2} + {8^2}} , + \sqrt {{6^2} + {8^2}} } \right] = [ - 10,10]$$
$$\alpha$$ = 210 & $$\beta$$ = 2$$-$$10
So, $$\alpha$$1/5 = 22 = 4
$$\Rightarrow$$ $$\beta$$1/5 = 2$$-$$2 = 1/4
quadratic 8x2 + bx + c = 0
$$ - {b \over 8} = {{17} \over 4}$$ $$ \Rightarrow $$ b = -34
$${c \over 8} = 1$$ $$ \Rightarrow $$ c = 8
$$ \therefore $$ c – b = 8 + 34 = 42
$$ = \max \{ {2^{6\sin 3x}}{.2^{8\cos 3x}}\} $$
$$ = max\{ {2^{6\sin 3x + 8\cos 3x}}\} $$
and $$\beta = \min \{ {8^{2\sin 3x}}{.4^{4\cos 3x}}\} = \min \{ {2^{6\sin 3x + 8\cos 3x}}\} $$
Now range of $$6sin3x + 8cos3x$$
$$ = \left[ { - \sqrt {{6^2} + {8^2}} , + \sqrt {{6^2} + {8^2}} } \right] = [ - 10,10]$$
$$\alpha$$ = 210 & $$\beta$$ = 2$$-$$10
So, $$\alpha$$1/5 = 22 = 4
$$\Rightarrow$$ $$\beta$$1/5 = 2$$-$$2 = 1/4
quadratic 8x2 + bx + c = 0
$$ - {b \over 8} = {{17} \over 4}$$ $$ \Rightarrow $$ b = -34
$${c \over 8} = 1$$ $$ \Rightarrow $$ c = 8
$$ \therefore $$ c – b = 8 + 34 = 42
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