Let P, Q, a and B be any four points, then PA Λ 2-pb Λ 2 = QA Λ 2-qb Λ 2 ＜ = > pq⊥ ab

Let P, Q, a and B be any four points, then PA Λ 2-pb Λ 2 = QA Λ 2-qb Λ 2 ＜ = > pq⊥ ab

(vector PA + vector Pb) (vector pa - vector Pb) = (vector QA + vector QB) (vector QA - vector QB)
Vector PA vector Pb = vector Ba
Vector QA vector QB = vector Ba
Vector PA vector QA = vector Pb vector QB = vector PQ
That is, 2 times vector PQ * vector Ba = 0
So PQ ⊥ ab

As shown in the figure, the line Mn is the axis of symmetry of line AB, point C is outside Mn, and Ca and Mn intersect at point D. if Ca + CB = 4cm, then the perimeter of △ BCD is equal to______ cm．

∵ DN is the vertical bisector of line AB, ∵ ad = BD, ∵ AC = AD + CD = BD + CD = AC, ∵ Ca + CB = 4cm, ∵ BCD circumference = BD + CD + BC = AD + CD + BC = Ca + BC = 4cm

What does the golden section mean?

Golden section point refers to dividing a line into two parts, so that the length of the original line is longer than that of the longer part. There are two such places on the line
Using the two golden section points on the line, we can make a pentagram and a Pentagon
The golden section is about 0.618:1
More than 2000 years ago, the third largest mathematician of Athenian School in ancient Greece, eudoxas, first proposed the golden section. The so-called golden section refers to dividing the line segment with length l into two parts, so that the ratio of one part to the whole is equal to that of the other part to that part