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Paradigm
Specialising in O Level Mathernatics
5 In the diagram, 
ABC is a triangle where 
M is the midpoint of 
AC.
The point 
P lies on the line 
BM such that 
BP=(1)/(3)BM.
The point 
Q lies on the line 
BC such that 
BQ:QC=1:4.
Given that 
vec(AB)=b and 
vec(AC)=c.
(a) Express, as simply as possible, in terms of 
b and/or 
c,
(i) 
vec(BC),
(ii) 
vec(BM),
(iii) 
vec(AP),
(iv) 
vec(AQ).
(b) Write down two facts about 
A,P and 
Q.
(c) Find the numeral value of 
(" Area of "/_\ABP)/(" Area of quadrilateral "CMPQ),
Ans:
(a)(i) 
c-b
(ii) 
(1)/(2)c-b
(iii) 
(1)/(6)(4b+c)
(iv) 
(1)/(5)(4b+c)
(b) 
A,P and 
Q are collinear 
AP:AQ=5:6
(c) 
(5)/(14)

Paradigm\newlineSpecialising in O Level Mathernatics\newline55 In the diagram, ABC A B C is a triangle where M M is the midpoint of AC A C .\newlineThe point P P lies on the line BM B M such that BP=13BM B P=\frac{1}{3} B M .\newlineThe point Q Q lies on the line BC B C such that BQ:QC=1:4 B Q: Q C=1: 4 .\newlineGiven that ABundefined=b \overrightarrow{A B}=b and M M 00.\newline(a) Express, as simply as possible, in terms of M M 11 and/or M M 22,\newline(i) M M 33,\newline(ii) M M 44,\newline(iii) M M 55,\newline(iv) M M 66.\newline(b) Write down two facts about M M 77 and Q Q .\newline(c) Find the numeral value of M M 99,\newlineAns:\newline(a)(i) AC A C 00\newline(ii) AC A C 11\newline(iii) AC A C 22\newline(iv) AC A C 33\newline(b) M M 77 and Q Q are collinear AC A C 66\newline(c) AC A C 77

Full solution

Q. Paradigm\newlineSpecialising in O Level Mathernatics\newline55 In the diagram, ABC A B C is a triangle where M M is the midpoint of AC A C .\newlineThe point P P lies on the line BM B M such that BP=13BM B P=\frac{1}{3} B M .\newlineThe point Q Q lies on the line BC B C such that BQ:QC=1:4 B Q: Q C=1: 4 .\newlineGiven that ABundefined=b \overrightarrow{A B}=b and M M 00.\newline(a) Express, as simply as possible, in terms of M M 11 and/or M M 22,\newline(i) M M 33,\newline(ii) M M 44,\newline(iii) M M 55,\newline(iv) M M 66.\newline(b) Write down two facts about M M 77 and Q Q .\newline(c) Find the numeral value of M M 99,\newlineAns:\newline(a)(i) AC A C 00\newline(ii) AC A C 11\newline(iii) AC A C 22\newline(iv) AC A C 33\newline(b) M M 77 and Q Q are collinear AC A C 66\newline(c) AC A C 77
  1. Vector BC Calculation: For BC\vec{BC}, since MM is the midpoint of ACAC, BC=BM+MC=BM+MA=BMAM\vec{BC} = \vec{BM} + \vec{MC} = \vec{BM} + \vec{MA} = \vec{BM} - \vec{AM}. Since AM=12AC\vec{AM} = \frac{1}{2}\vec{AC}, BC=BM12AC\vec{BC} = \vec{BM} - \frac{1}{2}\vec{AC}. But BM=BA+AM=AB+12AC=b+12c\vec{BM} = \vec{BA} + \vec{AM} = -\vec{AB} + \frac{1}{2}\vec{AC} = -b + \frac{1}{2}c. Therefore, BC=(b+12c)12c=b\vec{BC} = (-b + \frac{1}{2}c) - \frac{1}{2}c = -b.
  2. Vector BM Calculation: For BM\vec{BM}, since MM is the midpoint of ACAC, BM=BA+AM=AB+12AC=b+12c\vec{BM} = \vec{BA} + \vec{AM} = -\vec{AB} + \frac{1}{2}\vec{AC} = -b + \frac{1}{2}c.
  3. Vector AP Calculation: For AP\vec{AP}, since PP divides BMBM in the ratio 1:21:2 (BP:PM=1:2BP:PM = 1:2), \vec{AP} = \vec{AB} + \vec{BP} = \vec{AB} + \frac{\(1\)}{\(3\)}\vec{BM} = \mathbf{b} + \frac{\(1\)}{\(3\)}(-\mathbf{b} + \frac{\(1\)}{\(2\)}\mathbf{c}) = \mathbf{b} - \frac{\(1\)}{\(3\)}\mathbf{b} + \frac{\(1\)}{\(6\)}\mathbf{c} = \frac{\(2\)}{\(3\)}\mathbf{b} + \frac{\(1\)}{\(6\)}\mathbf{c}.
  4. Vector AQ Calculation: For \(\vec{AQ}\), since \(Q\) divides \(BC\) in the ratio \(1:4\) (\(BQ:QC = 1:4\)), \vec{AQ} = \vec{AB} + \vec{BQ} = \vec{AB} + \left(\frac{11}{55}\right)\vec{BC} = b + \left(\frac{11}{55}\right)(-b) = b - \left(\frac{11}{55}\right)b = \left(\frac{44}{55}\right)b.
  5. Collinearity of Points: For part (b), since PP lies on BMBM and QQ lies on BCBC, and MM is the midpoint of ACAC, it follows that AA, PP, and QQ are collinear. Also, since AP:AQ=BP:BQ=1:4AP:AQ = BP:BQ = 1:4, BMBM00.
  6. Area Ratio Calculation: For part (c), to find the area ratio, we use the fact that the areas of triangles with the same height are proportional to their bases. The area of triangle ABPABP is proportional to BPBP, and the area of quadrilateral CMPQCMPQ is the sum of the areas of triangles CMBCMB and CMQCMQ, which are proportional to BMBM and MQMQ, respectively. Since BP=13BMBP = \frac{1}{3}BM and MQ=45BCMQ = \frac{4}{5}BC, and BC=2BMBC = 2BM, BPBP00. The total base for the quadrilateral is BPBP11. Therefore, the area ratio is BPBP22. But we made a mistake, we forgot to consider the triangle CMBCMB in the area of the quadrilateral, which should be added to MQMQ to get the total base. So, the correct total base for the quadrilateral is BPBP55. Therefore, the correct area ratio is BPBP66. But we made another mistake, the correct ratio of BPBP to the total base of the quadrilateral is BPBP88. This is the correct area ratio.

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