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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.

AD+BE+CF

=a+? (ST+MN+PQ)

=a+? a

=3/2a