tanb=3，π& lt； b & lt3/2π

Available sinb=-(3√ 10)/ 10.

cosb=-(√ 10)/ 10

cos(a+b)=cosacosb-sinasinb

=( 1/2)*[-(√ 10)/ 10]-[-(√3)/2]*[- (3√ 10)/ 10]

=(-√ 10)/20-(3√30)/20

=-(√ 10+3√30)/20

2，k∈(- 1/2，0)a∈(-π/2，π/2)

sinacosa = k & lt0

Then a ∈ (-π/2,0)

In this range, Sina

Sina-COSA <; 0

1-2 Sina cosa = 1-2k( 1-2k & gt； 0)

(sina)^2-2sinacosa+(cosa)^2

=(sina-cosa)^2

= 1-2k

sina-cosa=-√( 1-2k)