In parallelogram ABCD, point K lies on side BC, BK: KC = 3: 5. express the vector DK in terms of the vectors BA = a and BC = b.

Since ABCD is a parallelogram, then its opposite sides are equal, then BA = CD.

Let the length of the segment KC = 5 * X, then BK = 3 * X.

ВK = 3 * KС / 5.

BC = KC + BK.

KC + 3 * KC / 5 = BC.

KC = 5 * 8 / BC.

Then | ↑ KС | = 5 * b / 8.

Vector | ↑ DK | = | ↑ DС | + | ↑ СK | = – | ↑ BA | – | ↑ KC | = -a – (5 * b / 8).



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