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AS Maths Modelling help please

Please help... AS Maths Pure Mathematics EDEXCEL Chapter 2, exercise 2H
A car manufacture uses a model to predict the fuel consumption, y miles per gallon(mpg), for a specific model of car travelling at a speed of x mph.
Y= -0.01x^2 + 0.975x+16, x>0
Question: use the model to find two speeds at which the car has a fuel consumption of 32.5mpg...
Answer: 21.8 and 75.7
THANK YOU
Place 32.5 as Y, so you get 32.5= -0.01x^2 + 0.975x+16, x>0,
Make it a quadratic equation so one side = 0, and solve using the quadratic formula.
Reply 2
Avatar for Musicanor
Musicanor
OP
9
Original post by Maxiimoose
Place 32.5 as Y, so you get 32.5= -0.01x^2 + 0.975x+16, x>0,
Make it a quadratic equation so one side = 0, and solve using the quadratic formula.

Yay thanks
i did that, and solved by the formula, bu t got -13.75 and 111.25, both of which are wrong.( correct answers are 28.1 and 75.7)can you please help me with finding out what i did wrong?
This thread is 3 years old and I have no idea what is about it but, from what your answers are you put the numbers incorrectly into the formula
Original post by Maxiimoose
This thread is 3 years old and I have no idea what is about it but, from what your answers are you put the numbers incorrectly into the formula

yeah, i had forgotten to place a negative sign.

Its funny how one sign can make such a difference.

thanks anyways though.
Reply 6
Avatar for Monitorme
Monitorme
1
That's exactly what I did but still got answers wrong.
32.5 =-0.01x*+0.975x+16>0

Then,
Using the formula
-0.975 +/- -0.975 squared -4*-0.01*16/2*-0.01
X=62.08
Reply 7
Avatar for davros
davros
16
Original post by Monitorme
That's exactly what I did but still got answers wrong.
32.5 =-0.01x*+0.975x+16>0

Then,
Using the formula
-0.975 +/- -0.975 squared -4*-0.01*16/2*-0.01
X=62.08


It's not a good idea to resurrect 5-year-old threads, but I've just done the calculation and got approx answers of 21.795 and 75.7 so the above posters were correct. I can't really decipher your post since the formatting is horrible, but did you remember to take the square root of b^2 - 4ac as per the formula?

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