Joseph Fourier developed a method for modeling any function with a combination of sine and cosine functions. You can graph this with your calculator easily and watch the modeling in action. Make sure you're in radian mode and let c=1:
f(x) = 4/(pi)sin(x) + 4/(3pi)sin(3x) + 4/(5pi)sin(5x) + 4/(7pi)sin(7x) + 4/(9pi)sin(9x) + 4/(11pi)sin(11x)
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