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2019-II-16

Posted on 16-06-202112-06-2023 By app.cch No Comments on 2019-II-16
Ans: D
Since $ABCD$ is a parallelogram, $BC = AD$ (opp. sides of //gram).

Since $\Delta BEX \sim \Delta DAX$,

$\begin{array}{rcl}
\dfrac{BX}{DX} & = & \dfrac{BE}{DA} \\
& = & \dfrac{BE}{BC} \\
& = & \dfrac{2}{12} \\
& = & \dfrac{1}{6}
\end{array}$

Consider $\Delta ABX$ and $\Delta ADX$. They have the same height if $BX$ and $DX$ are the bases respectively. Therefore, we have

$\begin{array}{rcl}
\dfrac{\text{the area of $\Delta ABX$}}{\text{the area of $\Delta ADX$}} & = & \dfrac{BX}{DX} \\
\dfrac{24}{\text{the area of $\Delta ADX$}} & = & \dfrac{1}{6} \\
\text{the area of $\Delta ADX$} & = & 144\text{ cm}^2
\end{array}$

Since $\Delta ADX \sim \Delta FBY$, we have

$\begin{array}{rcl}
\dfrac{\text{the area of $\Delta ADX$}}{\text{the area of $\Delta FBY$}} & = & \left(\dfrac{AD}{BY}\right)^2 \\
\dfrac{144}{\text{the area of $\Delta FBY$}} & = & \left(\dfrac{12}{9}\right)^2 \\
\text{the area of $\Delta FBY$} & = & 81 \text{ cm}^2
\end{array}$

Therefore, the area of the quadrilateral $CDYF$

$\begin{array}{cl}
= & \text{the area of $\Delta BCD$} – \text{the area of $\Delta BFY$} \\
= & \text{the area of $\Delta ABD$} – \text{the area of $\Delta BFY$} \\
= & \text{the area of $\Delta ABX$} + \text{the area of $\Delta ADX$} – \text{the area of $\Delta BFY$} \\
= & 24 + 144 – 81 \\
= & 87\text{ cm}^2
\end{array}$

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2019, HKDSE-MATH, Paper 2 Tags:Mensuration

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3D Problems (41) Basic Functions (13) Basic Geometry (68) Binomial Theorem (7) Change of Subject (32) Complex Numbers (16) Coordinates (46) Differentiation (16) Equations of Circle (54) Equations of Straight Line (43) Estimations and Errors (35) Factorization (39) Graph of Functions (3) Inequality (39) Integration (15) Laws of Indices (43) Linear Programming (21) Locus (13) Logarithm (34) Mathematical Induction (7) Matrices (4) Mensuration (98) Numeral System (19) Percentage (42) Polynomials (49) Probability (85) Properties of Circles (56) Quadratic Equations and Functions (57) Rate and Ratio (30) Rational Functions (20) Sequences (66) Simultaneous Linear Equations (27) Statistics (122) System of Linear Equations (3) Transformations (44) Trigonometry (M2) (7) Trigonometry and Its Applications (67) Variations (38) Vectors (3)

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