在△ABC中,a,b,c分别是内角A,B,C的对边,且a=4,C=2A,cosA=3/4. (Ⅰ)求sinB; (Ⅱ)求b的长.
在△ABC中,a,b,c分别是内角A,B,C的对边,且a=4,C=2A,cosA=
.
(Ⅰ)求sinB;
(Ⅱ)求b的长.
3 |
4 |
(Ⅰ)求sinB;
(Ⅱ)求b的长.
数学人气:173 ℃时间:2019-11-13 04:49:55
优质解答
(Ⅰ)在△ABC中,∵cosA=34,C=2A.∴cosC=cos2A=2cos2A−1=2•(34)2−1=18.从而sinA=74,sinC=378,∴sinB=sin(A+C)=sinAcosC+cosAsinC=74•18+34•378=5716.(Ⅱ)由正弦定理可得asinA=bsinB,∴b=...
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