数学解答题高中
\u6570\u5b66\u9ad8\u4e2d\u9898\u76ee\u89e3\u7b54 \u7b2c\u4e8c\u9898\u7b2c\u4e00\u95ee\uff1a
\u7531\u4f59\u5f26\u5b9a\u7406\u5f97\uff1aa^2=b^2+c^2-2bc cos(A).
A=60\u5ea6 cos(A)=1/2. b=c/3.
\u4ee3\u5165\u4e0a\u5f0f\u5f97\uff1a
a^2=(c/3)^2+c^2-(c/3)c=(7/9)c^2
a/c=sqrt(7)/3. (\u8fb9\u4e3a\u6b63\u53ea\u53bb\u6b63\u503c)\u3002
\u7b2c\u4e8c\u95ee\uff1a
\u7531\u6b63\u5f26\u5b9a\u7406: a/sin(A)=b/sin(B)=c/sin(C)
\u77e5sin(B)sin(C)/sin(A)^2=bc/a^2=1/3(c^2/a^2)=1/3\u00d7(7/9)=7/27
此图已经给出部分答案
角CAE=60——角A=120
根据勾股弦定理求出a——求出sinB——cosB——AAD可求
平面PAC垂直平面BAC且BO垂直AC
平面PAC上所有过O点的直线都垂直BO
证毕
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