.已知∠A、∠B、∠C是△ABC内角,求证:tanA/2 * tanB/2 + tanB/2 * tanC/2 + ta
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.已知∠A、∠B、∠C是△ABC内角,求证:tanA/2 * tanB/2 + tanB/2 * tanC/2 + tanC/2 * tanA/2 = 1
急
急
tanA/2×tanB/2+tanB/2×tanC/2+tanC/2×tanA/2
=tanA/2×tanB/2+tanC/2×(tanA/2+tanB/2)
=tanA/2×tanB/2+tan[90-(A+B)/2]×(tanA/2+tanB/2)
=tanA/2×tanB/2+cot(A/2+B/2)×(tanA/2+tanB/2)
=tanA/2×tanB/2+(tanA/2+tanB/2)/tan(A/2+B/2)
=tanA/2×tanB/2+1-tanA/2×tanB/2
=1
tanB/2=tan(π-A-C)/2=tan[π/2-(A+C)/2]=cot(A+C)/2
=(1-tanA/2×tanC/2)/(tanA/2+tanC/2)
因此tanA/2×tanB/2+tanB/2×tanC/2+tanA/2×tanC/2
=tanB/2(tanA/2+tanC/2)+tanA/2tanC/2
=[(1-tanA/2×tanC/2)/(tanA/2+tanC/2)]×(tanA/2+tanC/2)+tanA/2tanC/2
=1-tanA/2×tanC/2+tanA/2×tanC/2
=1
=tanA/2×tanB/2+tanC/2×(tanA/2+tanB/2)
=tanA/2×tanB/2+tan[90-(A+B)/2]×(tanA/2+tanB/2)
=tanA/2×tanB/2+cot(A/2+B/2)×(tanA/2+tanB/2)
=tanA/2×tanB/2+(tanA/2+tanB/2)/tan(A/2+B/2)
=tanA/2×tanB/2+1-tanA/2×tanB/2
=1
tanB/2=tan(π-A-C)/2=tan[π/2-(A+C)/2]=cot(A+C)/2
=(1-tanA/2×tanC/2)/(tanA/2+tanC/2)
因此tanA/2×tanB/2+tanB/2×tanC/2+tanA/2×tanC/2
=tanB/2(tanA/2+tanC/2)+tanA/2tanC/2
=[(1-tanA/2×tanC/2)/(tanA/2+tanC/2)]×(tanA/2+tanC/2)+tanA/2tanC/2
=1-tanA/2×tanC/2+tanA/2×tanC/2
=1
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