Answer:
Step-by-step explanation:

We know that : (a - b)(a + b) = a² - b²

We know that : 1 - sin²x = cos²x



We know that : sec²x = 1 + tan²x







I think it may be 1,680B but I'm not too sure...
Answer:
The first one
Step-by-step explanation:
The decimals line up when you subtract them
Answer:
7
Step-by-step explanation: calculate the prime factors of 7 and 21, than find the greatest common factor