yymaomaocc
幼苗
共回答了15个问题采纳率:93.3% 举报
a^2+b^2=6c^2
a^2+b^2-c^2=5c^2
cosC=(a^2+b^2-c^2)/(2ab)=5c^2/(2ab)
cosA=(b^2+c^2-a^2)/(2bc)
cosB=(a^2+C^2-b^2)/(2ac)
(cotA+cot B)tanC=(cosA/sinA+cosB/sinB)sinC/cosC
={(sinC/sinA)cosA+(sinC/sinB)cosB}/cosC
={ c/a cosA + c/b cosB } / {5c^2/(2ab)}
={ c/a (b^2+c^2-a^2)/(2bc)+ c/b (a^2+C^2-b^2)/(2ac) } / {5c^2/(2ab)}
={ (b^2+c^2-a^2)/(2ab)+ (a^2+C^2-b^2)/(2ab) } / {5c^2/(2ab)}
={ (b^2+c^2-a^2) + (a^2+C^2-b^2) } / {5c^2}
={ 2c^2} / {5c^2}
=2/5
1年前
9