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A mathematician friend mentioned to me that for small values of x we might closely approximate \sin(x) and \cos(x) by cubic polynomials. Was she correct? To answer this question, use your calculator in degree mode to try to find the coefficients a, \dots, h in the following suggested polynomial approximations: \sin(x) = a +bx+cx^2+dx^3\quad\quad \cos(x) = e+fx+gx^2+hx^3\quad\quad -0.1\leq x\leq 0.1
My mathematician friend now tells me that she thinks of trigonometrical functions in terms of solutions to differential equations, and not in terms of triangles. She says that \sin(x) and \cos(x) are both solutions to the second order differential equation: \frac{d^2 f}{dx^2}+f =0\;.
If the polynomial approximation continued to an arbitrarily high order, what would the coefficients be?
If you find the approximation to the sixth power of x you can now estimate trigonometrical values without using the \sin or \cos button on your calculator. Test the accuracy of your series for various values of x between 0 and \pi/2.
Discussion points: Do you think that your calculator stores values of sin and cos, or works them out on demand? Would your series provide an efficient way of evaluating the numerical values of sin(x) and cos(x)?
Two problems about infinite processes where smaller and smaller steps are taken and you have to discover what happens in the limit.