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2020-II-39

Posted on 16-06-202114-06-2023 By app.cch No Comments on 2020-II-39
Ans: A
Add a point $F$ on the circle $CDE$. Join $DF$ and $EF$.

Consider $\Delta CDF$.

$\begin{array}{rcll}
\angle CDF & = & \angle BCQ & \text{($\angle$ in alt. segment)} \\
\angle CDF & = & 35^\circ
\end{array}$

Hence, we have

$\begin{array}{rcll}
\angle ADE + \angle EDF + \angle CDF & = & 180^\circ & \text{(adj. $\angle$s on a st. line)} \\
100^\circ + \angle EDF + 35^\circ & = & 180^\circ \\
\angle EDF & = & 45^\circ
\end{array}$

Consider $\Delta ABC$.

$\begin{array}{rcll}
\angle BAC & = & \angle BCQ & \text{($\angle$ in alt. segment)} \\
\angle BAC & = & 35^\circ
\end{array}$

Consider $\Delta ADE$.

$\begin{array}{rcll}
\angle ADE + \angle AED + \angle DAE & = & 180^\circ & \text{($\angle$ sum of $\Delta$)} \\
100^\circ + \angle AED + 35^\circ & = & 180^\circ \\
\angle AED & = & 45^\circ
\end{array}$

Consider $\Delta DEF$.

$\begin{array}{rcll}
\angle DFE & = & \angle AED & \text{($\angle$ in alt. segment)} \\
\angle DFE & = & 45^\circ
\end{array}$

Hence, we have

$\begin{array}{rcll}
\angle DEF + \angle EDF + \angle EFD & = & 180^\circ & \text{($\angle$ sum of $\Delta$)} \\
\angle DEF + 45^\circ + 45^\circ & = & 180^\circ \\
\angle DEF & = & 90^\circ
\end{array}$

Consider quadrilateral $CDEF$.

$\begin{array}{rcll}
\angle DCF + \angle DEF & = & 180^\circ & \text{(opp. $\angle$s, cyc. quad.)} \\
\angle DCF + 90^\circ & = & 180^\circ \\
\angle DCF & = & 90^\circ
\end{array}$

Hence, consider $\Delta ABC$, we have

$\begin{array}{rcll}
\angle ABC + \angle ACB + \angle BAC & = & 180^\circ & \text{($\angle$ sum of $\Delta$)} \\
\angle ABC + 90^\circ + 35^\circ & = & 180^\circ \\
\angle ABC & = & 55^\circ
\end{array}$

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2020, HKDSE-MATH, Paper 2 Tags:Properties of Circles

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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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