Ruth Sin's answer to Kathy's Secondary 4 E Maths Singapore question.
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Be aware of converting units to ratio! It is inverted!
E.g., if a=2b, then a:b is 2:1, NOT 1:2!
E.g., if a=2b, then a:b is 2:1, NOT 1:2!
Date Posted:
4 years ago
The other way to find the ratio without inversion :
a = 4/3 b
So a/b = 4/3
This is like saying relative to each other,
a is 4 units and b is 3 units.
So a : b = 4 : 3
Similarly,
b = 5/6 c
b/c = 5/6
And likewise, b : c = 5 : 6
Now we need to make the units the same for both ratios.
To get the simplest ratio, we find the lowest common multiple of 3 and 5, which is 15.
(3 x 5 = 15 , 5 x 3 = 15)
a : b = 4 : 3 = 20 : 15
b : c = 5 : 6 = 15 : 18
So a : b : c = 20 : 15 : 18
The technique of making the units the same is actually utilised in PSLE math.
The expression of ratio in terms of fractions is also found in the topic of similar triangle in O level E Math
a = 4/3 b
So a/b = 4/3
This is like saying relative to each other,
a is 4 units and b is 3 units.
So a : b = 4 : 3
Similarly,
b = 5/6 c
b/c = 5/6
And likewise, b : c = 5 : 6
Now we need to make the units the same for both ratios.
To get the simplest ratio, we find the lowest common multiple of 3 and 5, which is 15.
(3 x 5 = 15 , 5 x 3 = 15)
a : b = 4 : 3 = 20 : 15
b : c = 5 : 6 = 15 : 18
So a : b : c = 20 : 15 : 18
The technique of making the units the same is actually utilised in PSLE math.
The expression of ratio in terms of fractions is also found in the topic of similar triangle in O level E Math
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