Question

5. Factorise a2- 21a + 108​

Answer 1

Answer:

this is the answer

first u need to see that the last number is the multiple of which numbers but the same should be the difference or total (add/subtract) of the middle number.

so in this sum 108 is the multiple of 9 and 12

and by adding 9+12 we get 21

Answer 2

Answer:

$a^2-21a+108\\= \left(a^2-9a\right)+\left(-12a+108\right)\\= a\left(a-9\right)-12\left(a-9\right)\\= \left(a-9\right)\left(a-12\right)$

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