# Trig Substitution

Trig substitution is the general strategy of substituting a trigonometric function for a given expression of another trigonometric function. It can be used to convert between polar coordinates and rectangular coordinates or to convert between cartesian coordinates and polar coordinates. Trig substitution is the replacement of a trigonometric function by a similar function. For instance, if one needs to convert from a polar coordinate to a rectangular coordinate, the trig substitution is to replace a sine by a cosine, or vice versa, in the given expression. If one needs to convert from a rectangular coordinate to a polar coordinate, the trig substitution is to substitute a cosine by a sine, or vice versa. If one needs to convert from a cartesian coordinate to a polar coordinate, the trig substitution is to substitute a sine or a cosine by a tangent. If one needs to convert from a polar coordinate to a cartesian coordinate, the trig substitution is to substitute a tangent by a sine or a cosine. If one needs to convert from a cartesian coordinate to a rectangular coordinate, the trig substitution is to substitute a sine or a cosine by a cosine, or vice versa.