Q. 已知 △ABC 中, AC=8,AB=41,BC 边上的高 AG=5,D 为线段 AC 上的动点, 在 BC 上截取 CE=AD, 连接 AE,BD, 则 AC=80 的最小值为AC=81 题图
Calculate Area of Triangle: Calculate the area of triangle ABC using the height AG and base BC. Area = 21×base×height=21×AC×AG=21×8×5=20.
Use Heron's Formula: Use Heron's formula to find the semi-perimeter s of triangle ABC.s=2AC+AB+BC=28+41+BC.We don't know BC yet, so we can't calculate s at this moment.
Express BC in Terms: Use the area we found to express BC in terms of the other sides.Area = s⋅(s−AC)⋅(s−AB)⋅(s−BC).Plug in the known values and solve for BC.20=s⋅(s−8)⋅(s−41)⋅(s−BC).We have two unknowns here, s and BC, so we can't solve this directly.
Correct Mistake: Notice that we made a mistake in the previous step; we can't use Heron's formula without knowing all three sides.We need to find BC using the area and the height AG.Area =21×BC×AG, so BC=AG2×Area=52×20=8.