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9514 1404 393
Answer:
y = 3.02x^3 -5.36x^2 +5.68x +8.66
Step-by-step explanation:
Your graphing calculator (or other regression tool) can solve this about as quickly as you can enter the numbers. If you have a number of regression formulas to work out, it is a good idea to become familiar with at least one tool for doing so.
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If you're trying to do this by hand, the x- and y-values give you 4 equations in the 4 unknown coefficients.
a·1^3 +b·1^2 +c·1 +d = 12
a·3^3 +b·3^2 +c·3 +d = 59
a·6^3 +b·6^2 +c·6 +d = 502
a·8^3 +b·8^2 +c·8 +d = 1257
Solving this by elimination, substitution, or matrix methods is tedious, but not impossible. Calculators and web sites can help. The solutions are a = 317/105, b = -75/14, c = 1193/210, d = 303/35. Approximations to these values are shown above.
To find this answer, we need to multiply 3.57 by 12.
Therefore, 12 planks will cost $42.84.
Therefore, 12 planks will cost $42.84.