The third question (2) on page 79 of the eighth grade mathematics excellent student training textbook
As shown in the figure, the points passing through O are parallel lines of three sides of an equilateral triangle. It is known that triangles OMN, OPQ and OST are equilateral triangles.
AD+BE+CF
=AS+SD+BM+ME+CP+PF
=(BM+MN+NC)+(SD+ME+PF) (parallelogram in the middle and equilateral triangle in the middle)
=a+？ (ST+MN+PQ)
And ST = OT = BM, PQ = OP = NC, ∴ ST+Mn+PQ = BM+Mn+NC = A.