I want to calculate surface area like this
Integrate[Boole[x^2 + y^2 < 1], {x, -1, 1}, {y, -1, 1}]
But Wolafram Mathematica said: invalid integration variable or limit(s) in {39/5, -1, 1}
What's the problem?
I looked at the official documentation but their examples give the same error. Moreover, it only works when adding a second integration variable.