It is known that in ladder ABCD, AD / / BC, point E is on AB, point F is on AC, and ad = a, BC = B Ask for: (1) Let E and f be the midpoint of AB and DC respectively, and verify: EF / / BC, and EF = half (a + b) (2) If: AE than EB = DF than FC = m than N, judge whether EF and BC are parallel, and use the algebraic expression of a, B, m, n to express EF, and prove. This problem can still be imagined without a picture, that is, an ordinary trapezoid has made a median line. The essence is to prove that the trapezoid median line is one-half of the trapezoid (upper bottom + lower bottom) and parallel to the bottom`

It is known that in ladder ABCD, AD / / BC, point E is on AB, point F is on AC, and ad = a, BC = B Ask for: (1) Let E and f be the midpoint of AB and DC respectively, and verify: EF / / BC, and EF = half (a + b) (2) If: AE than EB = DF than FC = m than N, judge whether EF and BC are parallel, and use the algebraic expression of a, B, m, n to express EF, and prove. This problem can still be imagined without a picture, that is, an ordinary trapezoid has made a median line. The essence is to prove that the trapezoid median line is one-half of the trapezoid (upper bottom + lower bottom) and parallel to the bottom`

(1) Using the triangle median line theorem (this is well proved by similar triangles), let AC be crossed by EF in the X triangle ABC and CAD, ex = BC / 2, XF = ad / 2, EF = ex + XF = 1 / 2 * (AD + BC) = (a + b) / 2 (2) make EE '/ / BC, let AC be crossed by EE' in the X triangle ABX AE / EB = ax / Xc, CAD XC / AX =

Please help solve a math problem: Thank you!
In a workshop, the original zinc and copper are 111 kg. Now to smelt zinc and copper into an alloy according to the ratio of 1:2, 9 kg of copper needs to be added. How many grams of original copper?

Total mass of alloy: 111 + 9 = 120 (kg)
Mass of copper in alloy: 120 / 3 = 40 (kg)
40-9 = 31 (kg)
A: there was 31 kg of copper

When a and B meet, they travel the same distance. When a arrives at B and B arrives at a, they immediately return to the original route. When a and B meet for the second time, they travel 1 km less than B. if there is an interval of 1 hour and 30 minutes between the first meeting and the second meeting, a and B will go back to each other, Find the velocity of water flow

1. 2. In the first meeting, if a and B travel the same distance and the same time, it means that a and B have the same speed, that is, a downstream speed = B upstream speed