Express 10x3−5x\frac{10x}{3-5x} in the form a+b3−5xa + \frac{b}{3-5x}, where aa and bb are integers. Hence, integrate 10x3−5x\frac{10x}{3-5x} with respect to xx.

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Worked solution

3(a)

1. Rewrite the numerator in terms of the denominator

We want to express 10x10x in terms of (3−5x)(3-5x).
10x=−2(−5x)=−2(3−5x−3)=−2(3−5x)+610x = -2(-5x) = -2(3 - 5x - 3) = -2(3 - 5x) + 6

2. Split the fraction and simplify to find $a$ and $b$

10x3−5x=−2(3−5x)+63−5x\frac{10x}{3-5x} = \frac{-2(3-5x) + 6}{3-5x}
=−2+63−5x= -2 + \frac{6}{3-5x}
Thus, a=−2a = -2 and b=6b = 6.

Answer

−2+63−5x-2 + \frac{6}{3-5x}

3(b)

1. Express the integrand using the result from part (a)

From part (a), we find aa and bb such that 10x3−5x=a+b3−5x\frac{10x}{3-5x} = a + \frac{b}{3-5x}.
10x=a(3−5x)+b=−5ax+(3a+b)10x = a(3-5x) + b = -5ax + (3a+b)
Comparing coefficients:
−5a=10  ⟹  a=−2-5a = 10 \implies a = -2
3a+b=0  ⟹  3(−2)+b=0  ⟹  b=63a + b = 0 \implies 3(-2) + b = 0 \implies b = 6
So, 10x3−5x=−2+63−5x\frac{10x}{3-5x} = -2 + \frac{6}{3-5x}.

2. Integrate the expression term by term

∫10x3−5x dx=∫(−2+63−5x) dx\int \frac{10x}{3-5x} \, dx = \int \left( -2 + \frac{6}{3-5x} \right) \, dx
=−2x+6−5ln⁡(3−5x)+c= -2x + \frac{6}{-5} \ln(3-5x) + c
=−2x−65ln⁡(3−5x)+c= -2x - \frac{6}{5} \ln(3-5x) + c

Answer

−2x−65ln⁡(3−5x)+c-2x - \frac{6}{5} \ln(3-5x) + c

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