I have a series of 5 known y values and five known x values. I have used the Excel in-built charting facility to plot a line graph / curve for these values, and have added a polynomial trend line projecting 6 further y values for a further series of known x's. I have enabled the chart option for the polynomial equation to be shown on the chart, which gives me the following:

y = 2.75x2 - 5.17x + 2.56.

The x and y values I am using are as follows:

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My issue is that when I use the equation to try to predict the next unknown y value by substituting x with the next actual x value (350) for which I am trying to forecast the y value, I get a nonsensical answer. i.e. I am computing y = ((2.75*(3502)) - (5.17*350)) + 2.56.

Please can anyone tell me why I do not get a sensible answer with this approach?

On a related question, I have tried to get Excel to show me the data for the projected y values by including a table underneath the line chart; however it only gives me the values for the known points and not the projected ones. Is there any way to choose an option so that Excel will show the data it is uisng to create the trendline plot?

Thank you in advance for your help.

The x and y values I am using are as follows:

x | y |

Metres | Drop (cms) |

100 | 0 |

150 | 3.5 |

200 | 11.8 |

250 | 25.6 |

300 | 45.6 |

350 | |

400 | |

450 | |

500 | |

550 | |

600 |

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My issue is that when I use the equation to try to predict the next unknown y value by substituting x with the next actual x value (350) for which I am trying to forecast the y value, I get a nonsensical answer. i.e. I am computing y = ((2.75*(3502)) - (5.17*350)) + 2.56.

Please can anyone tell me why I do not get a sensible answer with this approach?

On a related question, I have tried to get Excel to show me the data for the projected y values by including a table underneath the line chart; however it only gives me the values for the known points and not the projected ones. Is there any way to choose an option so that Excel will show the data it is uisng to create the trendline plot?

Thank you in advance for your help.