Thread: ratio of figures

i already proved the triangles.i need help with finding ratio of BXC to rectangle ABCD. it is given that CM=3MB

THANKS

Originally Posted by helloying

i already proved the triangles.i need help with finding ratio of BXC to rectangle ABCD. it is given that CM=3MB

THANKS

the ratio of the areas ?? Perhaps you mean CM=3MD ?

Pls clarify . Thanks .

Originally Posted by mathaddict

the ratio of the areas ?? Perhaps you mean CM=3MD ?

Pls clarify . Thanks .

sorry i left out the areas.i need to find the ratio of area of that triangle to the ractangle.CM=3MD is given from the question.

Produce a straight line where Y lies on AB, Z lies on DC, and YXZ//AD//BC.
CM = 3DM, hence
CM = 0.75*DC = 0.75*AB

hence
AB / CM
= AB / 0.75AB
= 4/3
= AX / CX
= BX / MX

triangle AYX similiar to CZX (AAA), therefore
AY / CZ = AX / CZ = 4/3
Therefore CZ = BY = (3/7)*AB

area of triangle BXC = BC*BY/2
= BC*(3/7*AB)*1/2
= (BC)(AB)(3/14)
area of rectangle ABCD = (BC)(AB)
Hence, the ratio concerned
= 3:14

Originally Posted by ukorov

Produce a straight line where Y lies on AB, Z lies on DC, and YXZ//AD//BC.
CM = 3DM, hence
CM = 0.75*DC = 0.75*AB

hence
AB / CM
= AB / 0.75AB
= 4/3
= AX / CX
= BX / MX

triangle AYX similiar to CZX (AAA), therefore
AY / CZ = AX / CZ = 4/3
Therefore CZ = BY = (3/7)*AB

area of triangle BXC = BC*BY/2
= BC*(3/7*AB)*1/2
= (BC)(AB)(3/14)
area of rectangle ABCD = (BC)(AB)
Hence, the ratio concerned
= 3:14

i think this AY / CZ = AX / CZ = 4/3 should be AY / CZ = AX / CX = 4/3

And i dont understand this step
Therefore CZ = BY = (3/7)*AB

By the way, this qn is only 1 mark.perhaps there a shorter method?

thanks

Originally Posted by helloying

i think this AY / CZ = AX / CZ = 4/3 should be AY / CZ = AX / CX = 4/3

And i dont understand this step
Therefore CZ = BY = (3/7)*AB

By the way, this qn is only 1 mark.perhaps there a shorter method?

thanks

ax /cx <--- oh yeah I typed it wrong

regarding to BY = (3/7)*AB :
first have a look at AY:BY again. it is 4:3. it means that WHEN AY is 4, BY is 3.....when AY is 20, BY is 15, etc.
it tells us that WHEN the whole length of AB is EQUALLY divided into 7 PARTS, then AY constitutes 4 parts, and BY consitutes 3.
therefore AY:BY = 4:3, AY:AB = 4:7, BY:AB = 3:7
These numbers do NOT have units (cm, mm, etc). it is all about ratios.
if you want to prove with real lengths, consider AY = 20cm, then BY = 15cm, and hence AB = 35cm. you will get the same ratios as aforesaid.

to shorten the calculation, it is not really necessary to have point Z and line XZ but you need Y and line BY which is equivalent to the height of triangle required to calculate its area. simply use the ratio of AX:CX to work out the ratios of AY:BY and AB:BY, and eventually present BY in terms of AB.