Q. Given the function f(x)=(−5x3−8x2−1)(9x2+1), find f′(x) in any form.Answer: f′(x)=
Product Rule Explanation: To find the derivative of the product of two functions, we use the product rule. The product rule states that the derivative of a product of two functions is the derivative of the first function times the second function plus the first function times the derivative of the second function.
Define Functions: Let's denote the first function as u(x)=−5x3−8x2−1 and the second function as v(x)=9x2+1. We need to find the derivatives u′(x) and v′(x).
Find Derivatives: First, we find the derivative of u(x). The derivative of −5x3 is −15x2, the derivative of −8x2 is −16x, and the derivative of a constant −1 is 0. So, u′(x)=−15x2−16x.
Apply Product Rule: Next, we find the derivative of v(x). The derivative of 9x2 is 18x, and the derivative of a constant 1 is 0. So, v′(x)=18x.
Expand Expressions: Now we apply the product rule: f′(x)=u′(x)v(x)+u(x)v′(x). Substituting the derivatives we found, we get f′(x)=(−15x2−16x)(9x2+1)+(−5x3−8x2−1)(18x).
Combine Like Terms: We need to expand the expressions to simplify them. First, we expand (−15x2−16x)(9x2+1), which gives us −135x4−15x2−144x3−16x.
Final Derivative: Next, we expand (−5x3−8x2−1)(18x), which gives us −90x4−144x3−18x.
Final Derivative: Next, we expand (−5x3−8x2−1)(18x), which gives us −90x4−144x3−18x.Now we combine like terms from both expansions. The terms −135x4 and −90x4 combine to −225x4. The terms −144x3 and −144x3 combine to −288x3. The terms −15x2 and −8x2 combine to −90x4−144x3−18x0. The terms −90x4−144x3−18x1 and −90x4−144x3−18x2 combine to −90x4−144x3−18x3.
Final Derivative: Next, we expand (−5x3−8x2−1)(18x), which gives us −90x4−144x3−18x.Now we combine like terms from both expansions. The terms −135x4 and −90x4 combine to −225x4. The terms −144x3 and −144x3 combine to −288x3. The terms −15x2 and −8x2 combine to −90x4−144x3−18x0. The terms −90x4−144x3−18x1 and −90x4−144x3−18x2 combine to −90x4−144x3−18x3.The final derivative −90x4−144x3−18x4 is then −90x4−144x3−18x5.
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