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JEE Main 202227 JulyEvening Shift
Exam
JEE Main
Year
2022
Date
27 July
Shift
Evening Shift
Subject
Mathematics
Chapter
Definite Integration

JEE Main 2022 · 27 July · Evening Shift · Mathematics · Definite Integration

∫ _ 0 ^ 2 ( |2 x ^ 2 - 3x| + [x - (1)/(2)])dx , where [t] is the...

{∫}_{0}^{2}\left(\left|2{x}^{2} - 3x\right| + \left[x - \dfrac{1}{2}\right]\right)dx,where[t], where \left[t\right] is the greatest integer function, is equal to

Options

  1. A76\dfrac{7}{6}
  2. B1912\dfrac{19}{12}
  3. C3112\dfrac{31}{12}
  4. D32\dfrac{3}{2}

Correct answer

B. 1912\dfrac{19}{12}

Step-by-step solution

∫02(∣2x2−3x∣+[x−12])dx{∫}_{0}^{2}\left(\left|2{x}^{2} - 3x\right| + \left[x - \dfrac{1}{2}\right]\right)dx

=∫02∣2x2−3x∣dx+∫02[x−12]dx= {∫}_{0}^{2}\left|2{x}^{2} - 3x\right|dx + {∫}_{0}^{2}\left[x - \dfrac{1}{2}\right]dx

Let I1=∫02∣2x2−3x∣dx=∫032(3x−2x2)dx+∫322(2x2−3x)dx{I}_{1} = {∫}_{0}^{2}\left|2{x}^{2} - 3x\right|dx = {∫}_{0}^{\dfrac{3}{2}}\left(3x - 2{x}^{2}\right)dx + {∫}_{\dfrac{3}{2}}^{2}\left(2{x}^{2} - 3x\right)dx

=(3x22−2x33)032+(2x33−3x22)322= {\left(\dfrac{3{x}^{2}}{2} - \dfrac{2{x}^{3}}{3}\right)}_{0}^{\dfrac{3}{2}} + {\left(\dfrac{2{x}^{3}}{3} - \dfrac{3{x}^{2}}{2}\right)}_{\dfrac{3}{2}}^{2}

=(278−94)+(83−6−94+278)=94−103=1912= \left(\dfrac{27}{8} - \dfrac{9}{4}\right) + \left(\dfrac{8}{3} - 6 - \dfrac{9}{4} + \dfrac{27}{8}\right) = \dfrac{9}{4} - \dfrac{10}{3} = \dfrac{19}{12}

Let I2=∫02[x−12]dx{I}_{2} = {∫}_{0}^{2}\left[x - \dfrac{1}{2}\right]dx

Assume x−12=tx - \dfrac{1}{2} = t

i.e. I2=∫−1232[t]dt{I}_{2} = {∫}_{\dfrac{- 1}{2}}^{\dfrac{3}{2}}\left[t\right]dt

=∫−120(−1)dt+∫010⋅dt+∫1321⋅dt= {∫}_{- \dfrac{1}{2}}^{0}\left(- 1\right)dt + {∫}_{0}^{1}0 \cdot dt + {∫}_{1}^{\dfrac{3}{2}}1 \cdot dt

=(−t)−120+(t)132=−12+(32−1)=0= {\left(- t\right)}_{- \dfrac{1}{2}}^{0} + {\left(t\right)}_{1}^{\dfrac{3}{2}} = - \dfrac{1}{2} + \left(\dfrac{3}{2} - 1\right) = 0

Hence, ∫02(∣2x2−3x∣+[x−12])dx=1912{∫}_{0}^{2}\left(\left|2{x}^{2} - 3x\right| + \left[x - \dfrac{1}{2}\right]\right)dx = \dfrac{19}{12}

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