Intensity of the superposition of sound waves

[chess10771]

Homework Statement

A sound wave with intensity 4 mW/m2 interferes destructively with a sound wave that has an intensity of 10 mW/m2. What is the intensity of the superposition of the two?

Homework Equations

sqrt of both values

The Attempt at a Solution

i believe that taking the sqrt of both values then adding them then squaring them is the correct solution but i keep getting the incorrect answer.

i keep getting 26.65

 

[ideasrule]

Why are you adding? The question says that the interference is destructive.

 

[chess10771]

ok so what exactly do i do then?

 

[Redbelly98]

Subtract, don't add, since the interference is destructive.