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**twostep08** suppose that 2 J of work is needed to stretch a string from its natural length of 30 cm to a length of 42 cm. how much work is needed to stretch it from 35 cm to 40 cm?

just when i thought i had this down, i got to this question (i got an exam tomorow, and the teacher just went over one easy example today, so i dont really have anything to reference)... this seems like im supposed to work backwards from the basic questions but im not at all sure how to tackle this.. preliminarily, i set up an integral from 0 to 12 that equals 2, but i dont know where to go at all from there... i would desperately appreciate it if someone can walk me through this one...

thanks so much!