利用定积分的几何意义,证明下列等式:-|||-(1) (int )_(0)^12xdx=1;-|||-(2) (int )_(0)^1sqrt (1-{x)^2}dx=dfrac (pi )(4);-|||-(3) (int )_(-pi )^pi sin xdx=0;-|||-(4) (int )_(-dfrac {pi )(2)}^dfrac (pi {2)}cos xdx=2(int )_(0)^dfrac (pi {2)}cos xdx.