Dear Uncle Colin,

How would you factorise $63x^2 + 32x - 63$? I tried the method where you multiply $a$ and $c$ (it gives you -3969) - but I’m not sure how to find factors of that that sum to 32!

Factors Are Troublesomely Oversized, Urgh

Hi, FATOU, and thanks for your message!

When the numbers in a quadratic like this get large (in mental arithmetic terms, at least), I try not to think about the number itself, but about its prime factorisation.

In this case, we know -3969 is $-63 \times 63$, making it $-3^2 \times 7 \times 3^2 \times 7$ or $-3^4 \times 7^2$.

### How does that help?

One approach is to go systematically through the possible factor splits like this:

• $\br{-1} \times \br{3^4 \times 7^2}$
• $\br{-3} \times \br{3^3 \times 7^2}$
• $\br{-7} \times \br{3^4 \times 7}$
• $\br{-3^2} \times \br{3^2 \times 7^2}$
• $\br{-3\times 7} \times \br{3^3 \times 7}$
• $\br{-3^3} \times \br{3 \times 7^2}$
• $\br{-7^2} \times \br{3^4}$
• $\br{-3^2 \times 7} \times \br{3^2 \times 7}$

(There are $(4+1)\times(2+1) = 15$ factors of $3^4 \times 7^2$ - we’ve found eight pairs, one of which is a ‘double’, so we have them all).

We could then add up each pair to see what we wind up with.

### But there’s a trick!

We know that 32 is not a multiple of 3, and not a multiple of 7. Any factor pair with a 3 in each factor, or a 7 in each factor, cannot possibly sum to 32! That reduces our workload considerably.

The only possibilities are the first - and $-1 + 3969$ is definitely not 32 - and the penultimate: $-49 + 81$ does indeed make 32.

(Had the middle number been a multiple of 3, or of 7, or of both, we could have used a similar idea: in those cases, the 3s, the 7s, or both must have been split across the factors.)

### Finishing it off

We can write the quadratic as $63x^2 + 81x - 49x - 63$, which is $9x(7x + 9) - 7(7x+9)$, or $(9x-7)(7x+9)$.

Hope that helps!
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 1, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0, "katex": 0, "math-container": 0, "wp-katex-eq": 0, "align": 0, "equation": 0, "x-ck12": 0, "texerror": 0, "math_score": 0.7499156594276428, "perplexity": 580.3726230486869}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 20, "end_threshold": 5, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "plain"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "warc_path": "s3://commoncrawl/crawl-data/CC-MAIN-2021-10/segments/1614178363217.42/warc/CC-MAIN-20210302034236-20210302064236-00008.warc.gz"}