The expression b^2 +10b +25 is the square of b+5. this means b^2+10b+25≡ (b+5)².

For factorization, rewrite b^2+10b+25 as b^2 +5b +5b+25. Take out common factors from the pairs b^2+5b and 5b+25 and rewrite it again as follows

=b(b+5b) +5(b+5)

Now again out the common factor from b(b+5) and 5(b+5) as follows

= (b+5)(b+5)

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