Question

how to get resultant intensity of two waves with different amplitudes

Answer 1

superposition

Answer 2

Since Intensity is proportional to amplitude squared: i is proportional A^2 then A is proportional to i ^(1/2). 1) Make phasor diagram using A values (not A^2 values!). 2) Now get "A(resultant) vector" using trig/algebra. 3) square the resultant amplitude to get the resultant intensity. Answer should match:
i(resultant) = i1 + i2 + 2(i1*i2)^(1/2) * cos(original given phase angle)