Question #96870
Differentiate from first principle square root of (x+√x).
1
Expert's answer
2019-10-21T10:28:34-0400
f(x)=x+xf(x)=\sqrt{x+\sqrt{x}}


f(x)=(x+x)=limh0f(x+h)f(x)h=f'(x)=\bigg(\sqrt{x+\sqrt{x}}\bigg)'=\lim\limits_{h\rarr0}{f(x+h)-f(x) \over h}=

=limh0x+h+x+hx+xh==\lim\limits_{h\rarr0}{\sqrt{x+h+\sqrt{x+h}}-\sqrt{x+\sqrt{x}} \over h}=

=limh0x+h+x+hx+xhx+h+x+h+x+xx+h+x+h==\lim\limits_{h\rarr0}{\sqrt{x+h+\sqrt{x+h}}-\sqrt{x+\sqrt{x}} \over h}\cdot{\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}} \over \sqrt{x+h+\sqrt{x+h}}}=

=limh0x+h+x+hxxh(x+h+x+h+x+x)==\lim\limits_{h\rarr0}{ x+h+\sqrt{x+h}-x-\sqrt{x}\over h(\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})}=


=limh0hh(x+h+x+h+x+x)+=\lim\limits_{h\rarr0}{ h\over h(\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})}+


+limh0x+hxh(x+h+x+h+x+x)=+\lim\limits_{h\rarr0}{ \sqrt{x+h}-\sqrt{x}\over h(\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})}=

=1x+0+x+0+x+x+={ 1\over \sqrt{x+0+\sqrt{x+0}}+\sqrt{x+\sqrt{x}}}+


+limh0x+hxh(x+h+x+h+x+x)x+h+xx+h+x=+\lim\limits_{h\rarr0}{ \sqrt{x+h}-\sqrt{x}\over h(\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})}\cdot{\sqrt{x+h}+\sqrt{x} \over \sqrt{x+h}+\sqrt{x}}=

=12x+x+limh0x+hxh(x+h+x+h+x+x)(x+h+x)=={ 1\over 2\sqrt{x+\sqrt{x}}}+\lim\limits_{h\rarr0}{ x+h-x\over h(\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})(\sqrt{x+h}+\sqrt{x})}=


=12x+x+limh01(x+h+x+h+x+x)(x+h+x)=={ 1\over 2\sqrt{x+\sqrt{x}}}+\lim\limits_{h\rarr0}{ 1\over (\sqrt{x+h+\sqrt{x+h}}+\sqrt{x+\sqrt{x}})(\sqrt{x+h}+\sqrt{x})}=

=12x+x+1(x+0+x+0+x+x)(x+0+x)=={ 1\over 2\sqrt{x+\sqrt{x}}}+{ 1\over (\sqrt{x+0+\sqrt{x+0}}+\sqrt{x+\sqrt{x}})(\sqrt{x+0}+\sqrt{x})}=

=12x+x(1+12x)={ 1\over 2\sqrt{x+\sqrt{x}}}\big(1+{1 \over 2\sqrt{x}}\big)



(x+x)=12x+x(1+12x)\bigg(\sqrt{x+\sqrt{x}}\bigg)'={ 1\over 2\sqrt{x+\sqrt{x}}}\bigg(1+{1 \over 2\sqrt{x}}\bigg)


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