(9x^2-1)/(1-6x+9x)=(9x^2-1)/(1+3x)=3x-1+0/(3x+1)=3x-1
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(2m-3n)² - 15n²+10mn
4m²-12mn+9n²-15n²+10mn=4m²-2mn-6n²=2(2m²-mn-3n²)
c^n+c^(n+2) + 1 +c² =c^n+c^n*c²+1+c²=c^n(1+c²)+(1+c²)=(1+c²)(c^n+1)