ABCDA1B1C1D1 -parallelepiped .D1C meets C1D at point M. Express the vector AM in terms of vectors AD1 and AC.

We construct additionally the vector D1K parallel to the vector AC and coinciding in direction, and the vector CК parallel to the vector AD1.

Then the vector AK = AD1 + D1K = AD1 + AC.

Quadrangle AD1KС is a parallelogram, since its opposite sides are parallel.

Then, by the property of the parallelogram, its diagonals, at the point of their intersection, are divided in half.

AM = AK / 2.

Then the vector AM = AK / 2 = (AD1 + AC) / 2.

Answer: Vector AM = (AD1 + AC) / 2.



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