Question

factor 3a^2-12ab-125b^2

Answer 1

the book gave me the answer 3(a-9b)(a+5b) i just don't know how they got there

Answer 2

Hmm... the book's answer doesn't match the question, it would give 135b^2.

Answer 3

your right its not 125 its 135

Answer 4

sorry

Answer 5

No prob. First notice you can factor out a 3 out of all the terms:\[\Large 3a^2-12ab-135b^2 = 3 (a^2-4ab -45b^2)\]

Answer 6

ok

Answer 7

Now try to factor whats inside the brackets. Don't worry so much about the a and b, just find two numbers that multiply to -45, and add up to -4.

Answer 8

You can always try listing the factors of 45 to see what will work
45 is 3*15, 5*9... -9 and +5 add up to -4.

Now you can factor it. You'll just have an a and a b in each one... since -9b*5b gives -45b^2
\[\Large 3 (a^2-4ab -45b^2) = 3 (a - 9b) (a+5b)\]

Answer 9

ah ok thank you, that makes sense.

Answer 10

Good :)