SOLUTION: I am a little confused on this problem and not sure where to start. Help would be greatly apprieciated. The polynomial 0.0014x^3-0.07x^2+1.5x+11 models natural gas consumption i

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Question 475083: I am a little confused on this problem and not sure where to start. Help would be greatly apprieciated.
The polynomial 0.0014x^3-0.07x^2+1.5x+11 models natural gas consumption in trillions of cubic feet, where x=0 corresponds to 1960, x=1 to 1961, and so on. In which year between 1979 and 1990 was natural gas consumption about 21.7 trillion cubic feet?

Thanks Abby
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
I'm not sure I understand this either.
:
The polynomial 0.0014x^3-0.07x^2+1.5x+11 models natural gas consumption in trillions of cubic feet,
where x=0 corresponds to 1960, x=1 to 1961, and so on.
In which year between 1979 and 1990 was natural gas consumption about 21.7 trillion cubic feet?
:
Write the given equation to equal 21.7
.0014x^3 - .07x^2 + 1.5x + 11 = 21.7
subtract 21.7 from both sides
.0014x^3 - .07x^2 + 1.5x + 11 - 21.7 = 0
:
.0014x^3 - .07x^2 + 1.5x - 10.7 = 0
:
Plot the equation:
+graph%28+300%2C+200%2C+-10%2C+50%2C+-30%2C+30%2C+.0014x%5E3+-+.07x%5E2+%2B+1.5x+-+10.7+%29+
You can see x ~ 13 yrs which would be 1973, This amt will not occur between 1979 and 1990