The minorities

Geometry Level 3

The area of the parallelograms AMSQ \text{AMSQ} , BNSM \text{BNSM} , CPTN \text{CPTN} , and DQTP \text{DQTP} are respectively 12, 36, 48, and 24.

What is the area of the part colored blue?


The answer is 5.

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1 solution

Let the height and base width of the big parallelogram A B C D ABCD be h h and b b respectively. Then the area [ A B C D ] = h b = 12 + 36 + 48 + 24 = 120 [ABCD] = hb = 12 + 36 + 48 + 24 = 120 .

Since A M S Q AMSQ and B N S M BNSM have the same height, Q S S N = \dfrac {QS}{SN} = [ A M S Q ] [ B N S M ] = \dfrac {[AMSQ]}{[BNSM]} = 12 36 = \dfrac {12}{36} = 1 3 \dfrac 13 Q S = 1 4 b \implies QS = \dfrac 14b . Similarly, Q T T N = \dfrac {QT}{TN} = [ D Q T P ] [ C P T N ] = \dfrac {[DQTP]}{[CPTN]} = 24 48 = \dfrac {24}{48} = 1 2 \dfrac 12 Q T = 1 3 b \implies QT = \dfrac 13b .

Now, we note that the area colored blue:

[ B T D S ] = 1 2 ( S T ) h = 1 2 ( Q T Q S ) h = 1 2 ( b 3 b 4 ) h = 1 2 1 12 b h Note that b h = 120 = 1 2 1 12 120 = 5 \begin{aligned} [BTDS] & = \frac 12 (ST) h \\ & = \frac 12 (QT-QS)h \\ & = \frac 12 \left(\frac b3 - \frac b4\right) h \\ & = \frac 12 \cdot \frac 1{12} \cdot \color{#3D99F6}bh & \small \color{#3D99F6} \text{Note that }bh = 120 \\ & = \frac 12 \cdot \frac 1{12} \cdot 120 \\ & = \boxed{5} \end{aligned}

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