∫20∫21(12x^3−27x^2.y^2)dydx
I = 20∫∫21(12x^3−27x^2.y^2)dydx = 20*21∫∫(12x^3−27x^2.y^2)dydx
I = 420∫(I2)dx, where I2 = ∫(12x^3−27x^2.y^2)dy
I2 = ∫(12x^3−27x^2y^2)dy = 12x^3.y – 9x^2.y^3
I2 = 12x^3.y – 9x^2.y^3
I = 420∫(I2)dx = 420∫(12x^3.y – 9x^2.y^3)dx
I = 420(3x^4.y – 3x^3.y^3)
I = 1260x^3.y(x – y^2)