Question

The term of a pendulum varies as the square root of it length if the length of a pendulum which beat 15 seconds find a: the length that beat 80 second b: find the time of a pendulum with length 36 centimeters

Answer 1

Please check what you type before posting it.
I think you mean
period = time for swing back and forth = T
T = k sqrt L = in fact 2 pi sqrt (L/g)
on earth g = about 9.81 m/s^2 and 2 pi = about 6.28
T = 6.28 sqrt (L/9.81)
if the period is 15 seconds then
15 = 6.28 sqrt (L/9.81)
sqrt (L/9.81) = 2.55
L/9.81 = 6.49
L = 63.7 meters
now if we say T = k sqrt L
15 = k sqrt 63.7
k = 1.88
so
T = 1.88 sqrt L
I think you may be missing a decimal point as well.