009A Sample Midterm 1, Problem 4 Detailed Solution

Let  $y={\sqrt {3x-5}}.$

(a) Use the definition of the derivative to compute   ${\frac {dy}{dx}}$   for  $y={\sqrt {3x-5}}.$

(b) Find the equation of the tangent line to  $y={\sqrt {3x-5}}$  at  $(2,1).$

---

Foundations:
1. Recall
$f'(x)=\lim _{h\rightarrow 0}{\frac {f(x+h)-f(x)}{h}}$
2. The equation of the tangent line to  $f(x)$  at the point  $(a,b)$  is
$y=m(x-a)+b$  where  $m=f'(a).$

Solution:

(a)

Step 1:
Let  $f(x)={\sqrt {3x-5}}.$
Using the limit definition of the derivative, we have
${\begin{array}{rcl}\displaystyle {f'(x)}&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {f(x+h)-f(x)}{h}}}\\&&\\&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {{\sqrt {3(x+h)-5}}-{\sqrt {3x-5}}}{h}}}\\&&\\&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {{\sqrt {3x+3h-5}}-{\sqrt {3x-5}}}{h}}.}\end{array}}$

Step 2:
Now, we multiply the numerator and denominator by the conjugate of the numerator.
Hence, we have
${\begin{array}{rcl}\displaystyle {f'(x)}&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {({\sqrt {3x+3h-5}}-{\sqrt {3x-5}})}{h}}{\frac {({\sqrt {3x+3h-5}}+{\sqrt {3x-5}})}{({\sqrt {3x+3h-5}}+{\sqrt {3x-5}})}}}\\&&\\&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {(3x+3h-5)-(3x-5)}{h({\sqrt {3x+3h-5}}+{\sqrt {3x-5}})}}}\\&&\\&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {3h}{h({\sqrt {3x+3h-5}}+{\sqrt {3x-5}})}}}\\&&\\&=&\displaystyle {\lim _{h\rightarrow 0}{\frac {3}{{\sqrt {3x+3h-5}}+{\sqrt {3x-5}}}}}\\&&\\&=&\displaystyle {\frac {3}{{\sqrt {3x-5}}+{\sqrt {3x-5}}}}\\&&\\&=&\displaystyle {{\frac {3}{2{\sqrt {3x-5}}}}.}\end{array}}$

(b)

Step 1:
We start by finding the slope of the tangent line to  $f(x)={\sqrt {3x-5}}$  at  $(2,1).$
Using the derivative calculated in part (a), the slope is
${\begin{array}{rcl}\displaystyle {m}&=&\displaystyle {f'(2)}\\&&\\&=&\displaystyle {\frac {3}{2{\sqrt {3(2)-5}}}}\\&&\\&=&\displaystyle {{\frac {3}{2}}.}\end{array}}$

Step 2:
Now, the tangent line to  $f(x)={\sqrt {3x-5}}$  at  $(2,1)$
has slope  $m={\frac {3}{2}}$  and passes through the point  $(2,1).$
Hence, the equation of this line is
$y={\frac {3}{2}}(x-2)+1.$

Final Answer:
(a)     Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle {\frac {dy}{dx}}={\frac {3}{2{\sqrt {3x-5}}}}}
(b)     Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y=\frac{3}{2}(x-2)+1}

Return to Sample Exam