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Direct Proportionality
What is it?
Direct proportionality is when one thing is
directly proportional to another.
Two quantities are in direct proportion if one
quantity increases at the same rate as the
other.…read more

Slide 3

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Y is proportional to X
· This is usually written as y x, or y=kx, where k
is the constant or proportionality.
Example
· A varies directly with b. Given that b is 10 when A is 35,
find a formula for A in terms of b.
· Then use this formula to find the value of A when b is 13.
· The value of b when A is 70.…read more

Slide 4

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Answer....
To work out this question, the following
calculation would be needed.
A b A = 3.5 x b
A=kxb = 3.5 x 13
35 = k x 10 = 45.5
k = 35 / 10
K = 3.5 70 = 3.5 x b
b = 70 / 3.5
A = 3.5 b = 20…read more

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Direct proportion involving x²
One quantity can also be proportional to the
square of another quantity.
Example
· Given the y varies directly with the square of x
and that y is 75 when x is 5, find y when x is 15.…read more

Slide 6

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Answer...
Y = kx² y = 3x²
75 = k x 5² y = 3 x 15²
75 = 25k y = 675
k=3…read more

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