# Math Help - Directly proportional

1. ## Directly proportional

D is directly proportional to the square of t

d=80 when t=4

so

d=kt^2

k being the constant

k=80/64

Work out the positive value of t when d=45

Can this be equated accurately without a calculator?

2. Originally Posted by Mukilab
D is directly proportional to the square of t

d=80 when t=4

so

d=kt^2

k being the constant

k=80/64 WRONG!! 4^2 = 16, NOT 64

Work out the positive value of t when d=45

Can this be equated accurately without a calculator?

D is directly proportional to the square of t does mean:

$d=k . t^{2}$

d=80 when t=4 gives you:

$80 = k \times (4)^2 \implies k = \frac{80}{16}=5$

Now can you find t when d=45?