✔ 最佳答案
By "definition of derivative" I'm assuming you mean one of the limit definitions, i.e. ƒ'(x) = lim(h→0) [(ƒ(x + h) − ƒ(x))/h]
Given:
ƒ(x) = √x
Apply the definition of derivative:
ƒ'(x) = lim(h→0) [(ƒ(x + h) − ƒ(x))/h]
= lim(h→0) [(√(x + h) − √x)/h]
Multiply by (√(x + h) + √x)/(√(x + h) + √x):
= lim(h→0) [(√(x + h) − √x)(√(x + h) + √x) / (h(√(x + h) + √x))]
FOIL out the numerator:
= lim(h→0) [(x + h − x) / (h(√(x + h) + √x))]
= lim(h→0) [h / (h(√(x + h) + √x))]
Cancel the h in the numerator and denominator:
= lim(h→0) [1 / (√(x + h) + √x)]
Let h go to 0:
= 1 / (√(x + 0) + √x)
= 1/(√x + √x)
= 1/(2√x)
Therefore, ƒ'(x) = 1/(2√x).