12.化下列二次积分为极坐标形式的二次积分:-|||-(1) (int )_(0)^1dx(int )_(0)^1f(x,y)dy;-|||-(2) (int )_(0)^2dx(int )_(x)^sqrt (3x)f(sqrt ({x)^2+(y)^2})dy;-|||-(3) (int )_(0)^1dx(int )_(1-x)^sqrt (1-{x^2)}f(x,y)dy;-|||-(4) (int )_(0)^1dx(int )_(0)^(x^2)f(x,y)dy.