To find the greatest common factor of two monomials, first find the prime factorization of each monomial, including all the variables (and a **–**1 factor if necessary).

Then take the product of all common factors.

**Example:**

Find the GCF of:

–27*p*^{2}*qr*^{5}

and

15*p*^{3}*r*^{3}

First, find the prime factorization of each monomial.

–27*p*^{2}*qr*^{5} = –1 · 3 · 3 · 3 · *p* · *p* · *q* · *r* · *r* · *r* · *r* · *r*

15*p*^{3}*r*^{3} = 3 · 5 · *p* · *p* · *p* · *r* · *r* · *r*

Common factors are shown in red. Their product is:

3 · *p *· *p* · *r *· *r* · *r*