1/cosA+1/cosC=-√2/cosB,A+C=2B,求cos[(A-C)/2]
来源:百度知道 编辑:UC知道 时间:2024/05/23 09:49:39
因A+B+C=π,又A+C=2B
得B=π/3
1/cosA+1/cosC=-2√2
=>(cosA+cosC)=-2√2cosAcosC
=>2cos(A-C)/2cos(A+C)/2=-√2[cos(A+C)+cos(A-C)]
=>cos(A-C)/2=-√2[-1/2+cos(A-C)]
=>cos(A-C)/2=-√2[-1/2+2cos²(A-C)/2-1]
=>4cos²(A-C)/2+√2cos(A-C)/2-3=0(|A-C|/2<π/3)
cos(A-C)/2=[-√2+√(2+48)]/8(负根舍去)
=√2/2
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已知(1+sina)/cosa=k,则cosa/(sina-1)的值是
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