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Identify the interval(s) on which the function y = 2x^2 - 8x - 10 is positive.
A. -1 B. x < -1 and x > 5
C. -5 D. X<-5 and x > 1


Sagot :

Given:

[tex]y=2x^2-8x-10[/tex]

To find:

The interval(s) on which the given function is positive.

Solution:

We have,

[tex]y=2x^2-8x-10[/tex]

[tex]y=2(x^2-4x-5)[/tex]

[tex]y=2(x^2-5x+x-5)[/tex]

[tex]y=2(x(x-5)+1(x-5))[/tex]

[tex]y=2(x+1)(x-5)[/tex]

For zeroes, [tex]y=0[/tex]

[tex]2(x+1)(x-5)=0[/tex]

[tex]x=-1,x=5[/tex]

-1 and 5 divide the number line in three parts [tex](-\infty , -1),(-1,5),(5,\infty )[/tex].

Interval        Selected Value      [tex]y=2(x+1)(x-5)[/tex]                      Sign

[tex](-\infty,-1)[/tex]                -2                  [tex]y=2(-2+1)(-2-5)=14[/tex]           +

[tex](-1,5)[/tex]                     0                   [tex]y=2(0+1)(0-5)=-10[/tex]              -

[tex](5,\infty)[/tex]                       6                  [tex]y=2(6+1)(6-5)=14[/tex]                 +

So, the function is positive for interval [tex](-\infty,-1)[/tex] and [tex](5,\infty)[/tex].

We can say that the function is positive for interval x < -1 and x > 5.

Therefore, the correct option is B.