Goal Seek: solve for an input

You know the answer you want — Goal Seek finds the input that produces it. Here: "what loan gives a $400/month payment?"

You'll need: abax only.

Run it

cd docs/examples/analysis/goal-seek
python run.py

What you should see

Borrowing 25000 costs 495.03/month
For a 400.00/month budget you can borrow 20200.80
Check: the sheet now shows payment = 400.00

How it works

• The sheet computes a payment with =-PMT(B2/12, B3*12, B1) — rate per month, number of months, principal (PMT returns a negative cash flow, so the leading - flips it for display).
• goal_seek(sheet, "B4", 400.0, "B1", lo=1000, hi=100000) varies B1 until B4 equals 400, writes the solution into the sheet, and returns it.
• On failure (target can't be reached inside lo..hi) the original cell value is restored and a GoalSeekError is raised — the sheet is never left half-solved.

Next steps

• In the GUI: Data → Goal seek does this from a dialog.
• For multi-scenario planning, the what-if scenario manager is covered in data & analysis tools.
• The financial family (PMT, PV, FV, NPV, IRR, bonds) is in the formula reference.