Select the approximate values of x that are solutions to f(x) = 0, where f(x) = -5x2 + 5x + 9

Question

A: {1.93, –0.93}

B: {-5, 5}

C: {–1.00, –1.80}

D: {–0.56, 0.56}

Answer 1

using the quadratic equation...

 

0=-5x²+5x+9

5x²-5x-9=0

 

[5+√(25+180)]/10

[5+√(205)]/10

(5+14.3178)/10

(5+14.3178)/10=19.3178/10=1.93

(5-14.3178)/10=-9.3178/10=-0.93

A:(1.93,-0.93) is the answer

Answer 2

     When you plug a value into a function, you get the result of whatever the function did to the x value that was substitute. In your example, f(3)=-21 and f(7)=-201 .... Another way of writing this is to say that the graph goes through (3,-21) and (7,-201). So according to your choices, you are trying to find when f(x)=0 or the points closest to(  ,0) In this case both choices a and d have an x value that when plugged in, result in a y near 0. This makes sense since this is a quadratic equation which should have 2 answers.

 

By the way, you could have substituted in 0 for f(x) and got 0=-5x^2+5x+9. This requires the quadratic formula to solve, which I dont know if you know, but you can find the exact answers.