In trapezoidal ABCD, AD / / BC, ∠ B = 40 ° and ∠ C = 50 °, e, m, F and N are the midpoint of AB, BC, CD and Da respectively, and EF = a, Mn = B 6. In trapezoidal ABCD, AD / / BC, ∠ B = 40 ° and ∠ C = 50 °, e, m, F and N are the midpoint of AB, BC, CD and ad respectively, and EF = a, Mn = B, then what is the length of BC? (expressed by the algebraic formula of a and b)! It's a process! hurry up!

BC=tan50*b+tan40*b+AD
And because AD + BC = 2A
Substituting ad = 2a-bc into the above formula
The results show that BC = (tan40 + tan50) * B / 2-a

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