It is known that in isosceles △ ABC, ab = AC. if the acute angle of the intersection of the vertical bisector of AB and the line of edge AC is 40 degrees, then the base angle of isosceles △ ABC ∠ B is 40 degrees______ ．

① When de intersects with line AC, as shown in Figure 1, ∵ De is the vertical bisector of AB, ∵ AED = 40 °, ∵ a = 90 ° - AED = 90 ° - 40 ° = 50 °, ∵ AB = AC, ∵ ABC = 12 (180 ° - a) = 12 (180 ° - 50 °) = 65 °; when de intersects with the extension of Ca, as shown in Figure 2, ∵ De is the vertical bisector of ab

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