Logarithmic spiral

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It can be described in polar coordinates (r,θ) by the equation

r = ae^{b\theta}\,

with real numbers a and b.

The JavaScript code to produce this picture

board = JXG.JSXGraph.initBoard('jsxgbox', {boundingbox: [-10, 10, 10, -10]});
var a = board.createElement('slider', [[1,-1],[5,-1],[0,0.3,1]],{name:'a'});
var b = board.createElement('slider', [[1,-2],[5,-2],[0,0.15,1]],{name:'b'});
var c = board.createElement('curve', [function(phi){return a.Value()*JXG.Math.pow(Math.E,b.Value()*phi); }, [0, 0],0, 8*Math.PI],
{curveType:'polar', strokewidth:4});
var g = board.createElement('glider', [c]);
var t = board.createElement('tangent', [g], {dash:2,strokeColor:'#a612a9'});
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