tana＋tanb=－5/3、tanatanb=1/3
(1)cot(a＋b)=1/[tan(a＋b)]=[1－tanatanb]/[tana＋tanb]=－2/5
(2)tan²a＋tan²b=(tana＋tanb)²－2tanatanb=19/9