# Surds - addition and and subtraction

• Nov 20th 2009, 09:59 PM
BeVs
Surds - addition and and subtraction
Simplify:

$6\sqrt{44}+4\sqrt{120}-\sqrt{99}-3\sqrt{120}$

$12\sqrt{11}+8\sqrt{30}-3\sqrt{11}-3\sqrt{30}$

$9\sqrt{11}+5\sqrt{30} \ \ \ \ \ \ \ \text{:my answer}$

$9\sqrt{11}+\sqrt{30} \ \ \ \ \ \ \ \ \text{:text book answer}$

Where have I gone wrong!

.
• Nov 20th 2009, 10:31 PM
Unenlightened
Quote:

Originally Posted by BeVs
Simplify:

$6\sqrt{44}+4\sqrt{120}-\sqrt{99}-3\sqrt{120}$

$12\sqrt{11}+8\sqrt{30}-3\sqrt{11}-3\sqrt{30}$

$9\sqrt{11}+5\sqrt{30} \ \ \ \ \ \ \ \text{:my answer}$

$9\sqrt{11}+\sqrt{30} \ \ \ \ \ \ \ \ \text{:text book answer}$

Where have I gone wrong!

Obviously the discrepancy is with your $-3\sqrt{270}$ part.
This equals $-3\sqrt{9}\sqrt{30} = -3*3\sqrt{30} = -9 \sqrt{30}$
So then your $+8\sqrt{30}-9\sqrt{30} = -\sqrt{30}$
So you should have $9 \sqrt {11}- \sqrt {30}$...

Which is different from the text book answer you gave... are you sure you took it down correctly?
• Nov 21st 2009, 12:04 AM
BeVs
ah many thanks, yeh sry, its a - sign at the end ^.^