What value (s) of ϑ, in radians make (s) x minimum?

He relationship between rectangular and rotational coordinates can be represented by the vector equation: r = xi + yj = rcos (theta) i+rsin (theta) j What values in radians makes x minimum y maximum and x = - y Ans : x minimum = - r when theta = 180 degrees + any multiple of 360 degrees y minimum = - r when theta = 270 degrees + any multiple of 360 degrees x = - y when theta = 135 degrees + any multiple of 180 degrees.