1=sin^2a+cos^2a
=(sina+cosa)^2-2sinacosa
=k^2-2(k+1)
=k^2-2k-2
k^2-2k-3=0
(k-3)(k+1)=0
k=3或-1
當k=3時,x^2-3x+4=0,△<0,所以舍去
當k=-1時,x^2+x=0
x=0或-1
當sina=0,cosa=-1時,a=π
當cosa=0,sina=-1時,a=3π/2
所以k=-1 a=π或3π/2