1+tanA+1+tanB |
(1+tanA)(1+tanB) |
=
1+tanA+tanB+1 |
1+tanA+tanB+tanAtanB |
又A+B>90°,
∴90°>A>90°-B>0
∴tanA>tan(90°-B)=cotB>0
∴tanA•tanB>1,
∴S<1
(2)
tanA |
1+tanA |
tanB |
1+tanB |
=
tanA+tanA•tanB+tanB+tanA•tanB |
(1+tanA)(1+tanB) |
1+tanA+tanB+1 |
(1+tanA)(1+tanB) |
=
2(tanA•tanB−1) |
(1+tanA)(1+tanB) |
∴S<
tanA |
1+tanA |
tanB |
1+tanB |