How can I write the equation of this graph?

Question

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These are points on the graph, so can we write a equation for the graph?

A(0,30), B(10,52), C(20,38), D(30,0), E(40,-37), F(50,-53)

Answer 1

Assuming it is in the form

$$ y = A \cos \left( 2 \pi x/ \lambda - \phi\right ) $$

or in degrees

$$ y = A \cos \left( 360 x/ \lambda - \phi \, 180/ \pi\right) $$

it requires only 3 points.By least squares best fit can be found.

$$ A= 53 , \lambda= 80, \phi = 20^0 $$