19.解:(1)由题意知,I0A=10M|=1,点A(cos,sin),则有S,oAw=210Ml·sina=5,解得sna=255,又为锐角,则72na=5,因钝角阝的终边与单位圆0的交占B的:则c0sB=-7沿mB=1-co3B=810,所以cos(a-)=co+sinasi=-亭×(-7+2×治-10(2)由(1)知sinu=25cosavs cosa sinp 10co=7√25.10,则smta-)=-sineeosB--cosusinB-25x(-2-x号-,10,从而sin(2a-B)-sinla+(singeos(a-B)+osasin(aB)x()+×(-9-号2因为为锐角,snu=>受,则有ae(周.即2ae(列,8e(。因此2a-B∈(-,所以2a-B=-.20.(1)设甲答对题目的数目为5则5~B(4,0.5),X=105-5(4-5)=155-20,即E(X)=15E(5)-20=15×4×0.5-20=10D(X)=152×D(5)=15×15×4×0.5×0.5=225(2)设乙答对的题目数为7,则Y=10n-5(4-7)=157-20,Y各个取值的概=4率PY=-20)=P=0)C=2102PY=-)=P0=1)=4C035PW=10)=P7-2)-CC=3,PY=25)=PW=3)=c8-87C621P(Y=40)=P(7=4)=14所以7的分布列为:上-20-5102540P14381210352114