Q18:
Which of the following is equal to (2k)(5k − 1)?
A, 2(10k − 1)
B. 5(10k − 1)
C. 10k
D. 2(10k )
E. 102k − 1
it seems like there is a formula..but i cannot remember it at all, 请指點一下!
Answer is A
many thanks !!!!
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