Question

What’s the least common multiple of x^2-2x-15 and x^2+2x-3?

Answer 1

`\large\boxed{LCM(x^2-2x-15,\ x^2+2x-3)=(x+3)(x-5)(x-1)}\\\boxed{=x^3-3x^2-13x+15}`

Explanation:

`x^2-2x-15=x^2+3x-5x-15=x(x+3)-5(x+3)=(x+3)(x-5)\\\\\\x^2+2x-3=x^2+3x-1x-3=x(x+3)-1(x+3)=(x+3)(x-1)\\\\LCM(x^2-2x-15,\ x^2+2x-3)=(x+3)(x-5)(x-1)\\\\=(x^2-2x-15)(x-1)\qquad\text{use FOIL}\\\\=(x^2)(x)+(x^2)(-1)+(-2x)(x)+(-2x)(-1)+(-15)(x)+(-15)(-1)\\\\=x^3-x^2-2x^2+2x-15x+15\qquad\text{combine like terms}\\\\=x^3+(-x^2-2x^2)+(2x-15x)+15\\\\=x^3-3x^2-13x+15`