Question
Find the interval of x where the expression 21x - 3x² - 54
is greater than zero.Solution
ATQ,
So, 21x - 3x² - 54 > 0
Or, 3x² - 21x + 54 < 0
Or, 3x² - 18x - 3x + 54 < 0
Or, 3x(x - 6) -3(x - 6) < 0
Or, (3x - 3)(x - 6) < 0
So, x = 1 and x = 6
Therefore, the expression is positive between 1 and 6.
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