# 20 waves of amplitudes 1 , 2 ...... , 9 , 20 mts are interfered . Maximum amplitude of the resulting wave is

$(a)\;210\;mts\qquad(b)\;300\;mts\qquad(c)\;200\;mts\qquad(d)\;250\;mts$

Answer : (a) $\;210\;mts$
Explanation :
Maximum amplitude is $\;A_{1}+A_{2}+......A_{20}$
$=1+2+.......+20$
$=\large\frac{21\times20}{2}=210\;mts$
It occurs when they are in same phase .