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C. 10x

Step-by-step explanation:

Well since there is 10 envelopes per box, and he buys and unknown number of boxes, you will be multiplying. Therefore A connot be an answer because it is addition. B cannot be an answer because it is division. And, D cannot be an answer because it is subtraction.

a4 - 3a2 - 1

step-by-step explanation:

step   1   :

equation at the end of step   1   :

  ((a4) -   3a2) -   1

step   2   :

trying to factor by splitting the middle term

2.1     factoring   a4-3a2-1  

the first term is,   a4   its coefficient is   1 .

the middle term is,   -3a2   its coefficient is   -3 .

the last term, "the constant", is   -1  

the answer is c: divide the arc's degree measure by 360°, then multiply by the circumference of the circle.