Resultant amplitude for waves of different frequencies?

[mintkints]

Homework Statement

Two superimposing waves are represented by equations
y1= 2sin 2π(10t – 0.4x)

y2= 4sin 2π(20t – 0.8x)

Find the ratio of Imax and Imin. Ans is (25:9)

Homework Equations

The Attempt at a Solution

I know that intensity is proportional to the square of amplitude and

maximum amplitude = a1 + a2

minimum amplitude = a1 - a2

but these equations are only applicable for waves with same frequency. so how do I proceed ? Thanks in advance!

 

[haruspex]

how do I proceed ?

Consider some fixed point x along the wave. How would you find the net amplitude at that point?