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To factor a trinomial in the form Ax^2 + Bx + C when the A = 1.  You need to find the factors of C that when added or subtracted will give you B.  

Factors of 21 are either 1,21 or 3,7.  Since +3 and +7 will give you 10 then those will be the two factors you will use.

Factoring x^2+10x+21 will give you (x+3)(x+7)

The factorization of [tex]\rm x^2+10x+21[/tex] is [tex]\rm (x+7)(x+3)[/tex].

Factorization;

Factorization is the process of writing a polynomial into its small factors.

The given polynomial is

[tex]\rm x^2+10x+21[/tex]

To find the factor of the polynomial follows all the steps given below.

The factor of the polynomial is

[tex]\rm x^2+10x+21\\\\x^2+7x+3x+21\\\\x(x+7)+3(x+7)\\\\(x+7)(x+3)[/tex]

Hence, the factorization of [tex]\rm x^2+10x+21[/tex] is [tex]\rm (x+7)(x+3)[/tex].

To know more about factorization click the link given below.

https://brainly.com/question/26354419