(1)(sina+cosa)^2=sina^2+cosa^2+2sinaXcosa =1+2sinaXcosa =(7/13)^2
2sinaXcosa =(7/13)^2-1
sinaXcosa =((7/13)^2-1)/2=-60/169
(2)(sina-cosa)^2=sina^2+cosa^2-2sinaXcosa =1-2sinaXcosa=1-(-120/169)=289/169
sina-cosa=(289/169)^(1/2)
(3)sina+cosa=7/13,sina-cosa=(289/169)^(1/2)联立解出sina,cosa,tana=sina/cosa