Question

问一道向量题我要第7题,B

Answer 1

∵p//q

∴(a+c)(c-a)=b(b-a)

ac+c^2-a^2-ac=b^2-ab

c^2-a^2-b^2=-ab

a^2+b^2-c^2=ab

由余弦定理

a^2+b^2-c^2=2abcosC

∴2abcosC=ab

cosC=1/2

C=π/3