b*c=|b|。 |c|cos(b，c)=|b|。 |c|。 (- 1/3)

a*c=|a|。 |c|cos(a，c)

As can be seen from a+b+c=0, vectors A, B and C form an end-to-end triangle, which is a solution triangle in common English.

Let vectors A, B and C correspond to three sides A, B and C of a triangle respectively, and the corresponding angles are also Angle A, Angle B and Angle C..

Then cos angle C= 1/5 cos angle A=- 1/3, then cos angle b can be obtained from angle A+ angle B+ angle C= 180 degrees.

Find the Cos angle b, because the length of the B side of the triangle is = 1, so you can find the combination angle.

Face a and face c, so that a*c=|a|. |c|cos angle b solution.

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