Hello!
First, I would like address question 1. I think it would be appropriate to say, "rationalize the radicand" instead of the "denominator", because these questions are dealing with radicals, not fractions.
2A. √40 = √4 · √10 = 2√10. This is correct, and the estimation kind of off, the square root fo 10 is actually about 3.1 to 3.2, and since it's multiplied by 2, it's around 6.2 to 6.3. It's closer to 6.3.
2B. √75 = √25 · √3 = 5√3. Since the square root of 3 is about 1.6 to 1.7, and it's multiplied by 5, it's estimation is about 8.6 to 8.7. It's closer to 8.7.
2C. √90 = √9 · √10 = 3√10. As above, the square root fo 10 is actually about 3.1 to 3.2, and since it's multiplied by 3, it's around 9.4 to 9.5. It's closer to 9.5.
2D. √245 = √49 · √5 = 7√5. The square root of 5 is about 2.2 to 2.3, and since it's multiplied by 7, it can be estimated to be about 15.6 to 15.7. It's close to 15.7.
2E. √450 = √255 · √2 = 15√2. The square root of 2 is about 1.4 to 1.5, and since it's multiplied by 15, it's estimated to be about 21.1 to 21.2.
2F. √640 = √64 · √10 = 8√10. As stated above, the square root fo 10 is actually about 3.1 to 3.2, and since it's multiplied by 8, its estimation is around 25.3 to 25.4. It's closer to 25.3.
2G. √675 = √225 · √3 = 15√3. Since the square root of 3 is about 1.6 to 1.7, and it's multiplied by 15, it's estimated to be about 25.9 to 26.
These questions can be solved if you know your perfect squares. Remember those, and you'll be fine. :^)
