f(x)÷(-5x^2)
=(10x^2+8x^5-12x^3)÷(-5x^2)
=-2-5x^3/5+12x/5
f(x)×(x^2-1)
=(10x^2+8x^5-12x^3)(x^2-1)
=10x^4-10x^2+8x^7-8x^5-12x^5+12x^3
=10x^4-10x^2+8x^7-20x^5+12x^3
f(x)÷(-5x^2)
=(10x^2+8x^5-12x^3)÷(-5x^2)
=-2-5x^3/5+12x/5
f(x)×(x^2-1)
=(10x^2+8x^5-12x^3)(x^2-1)
=10x^4-10x^2+8x^7-8x^5-12x^5+12x^3
=10x^4-10x^2+8x^7-20x^5+12x^3