看不懂的数学符号
幼苗
共回答了13个问题采纳率:100% 举报
(1) 2cos(A+B)+cos2C=-3/2
2cos(π-C)+2cos^2C-1=-3/2
-2cosC+2cos^2C=-1/2
2(cosC-1/2)^2=-1/2+1/2
(cosC-1/2)^2=0
cosC=1/2
C=π/3
(2) a+b=9
b=9-a
c^2=a^2+b^2-2abcosC
a^2+b^2-2ab*1/2=39
a^2+(9-a)^2-a(9-a)=39
a^2+81-18a+a^2-9a+a^2=39
3a^2-27a+42=0
a^2-9a+14=0
(a-2)(a-7)=0
a1=2
a2=7
b1=9-2=7
b2=9-7=2
因此,设a=2,b=7
SΔABC=1/2a*bsinC
=1/2*2*7sinπ/3
=7√3/2
1年前
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