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小 發表於 2011-8-7 05:36 PM (第 4663 天)
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回覆 9# dororo988 的帖子
1.
dy/dx
= lim {[3-2√(x+h)]-[3-2√(x)]}/h
= 2 * lim [√x-√(x+h)]/h
= 2 * lim [√x-√(x+h)]/h * [√x+√(x+h)]/[√x+√(x+h)]
= 2 * lim (-1)/[√x+√(x+h)]
= 2 * (-1)/(2√x)
= -1/√x
2.
dy/dx
= lim {[4√(x+h)-1]-[4√(x)-1]}/h
= 4 * lim [√(x+h)-√(x)]/h
= 4 * lim [√(x+h)-√(x)]/h * [√(x+h)+√(x)]/[√(x+h)+√(x)]
= 4 * lim 1/[√(x+h)+√(x)]
= 4 * 1/(2√x)
= 2√x