미적분Ⅰ미분계수와 도함수수능 기출발전 문제 (3점 후반~4점 초반)

절댓값 함수 미분계수

문제

함수 f(x)=xx+x13f ( x ) = x | x | + | x - 1 | ^{3}에 대하여 f(0)+f(1)f' ( 0 ) + f' ( 1 )의 값은? [4점] 3- 31- 1113355

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아직 올라온 파일이 없습니다.

해설

g(x)=xxg ( x ) = x | x |, h(x)=x13h ( x ) = | x - 1 | ^{3}으로 놓으면 두 함수 g(x)g ( x ), h(x)h ( x )는 실수 전체에서 미분가능하므로 함수 f(x)f ( x )도 실수 전체에서 미분가능하다. g(0)=0g' ( 0 ) = 0, h(1)=0h' ( 1 ) = 0이고 x>0x > 0일 때 g(x)=x2g ( x ) = x ^{2}, x<1x < 1일 때 h(x)=(x1)3h ( x ) = - ( x - 1 ) ^{3}이므로 f(0)=0+h(0)f' ( 0 ) = 0 + h' ( 0 )=3(01)2=3= - 3 ( 0 - 1 ) ^{2} = - 3 f(1)=g(1)+0=21=2f' ( 1 ) = g' ( 1 ) + 0 = 2 \cdot 1 = 2 \therefore f(0)+f(1)=3+2=1f' ( 0 ) + f' ( 1 ) = - 3 + 2 = - 1 [다른풀이1] f(x)=xx+x13f ( x ) = x | x | + | x - 1 | ^{3}에서 f(0)=1f ( 0 ) = 1, f(1)=1f ( 1 ) = 1 limh+0f(0+h)f(0)h\displaystyle \lim\limits _{h \rightarrow + 0} \frac{f ( 0 + h ) - f ( 0 )}{h}=limh+0hh+h131h\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{h | h | + | h - 1 | ^{3} - 1}{h} =limh+0h2(h1)31h\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{h ^{2} - ( h - 1 ) ^{3} - 1}{h}=limh+0h3+4h23hh\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{- h ^{3} + 4 h ^{2} - 3 h}{h} =limh+0(h2+4h3)\displaystyle = \lim\limits _{h \rightarrow + 0} {} ( - h ^{2} + 4 h - 3 )=3= - 3 limh0f(0+h)f(0)h\displaystyle \lim\limits _{h \rightarrow - 0} {} \frac{f ( 0 + h ) - f ( 0 )}{h}=limh0hh+h131h\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{h | h | + | h - 1 | ^{3} - 1}{h} =limh0h2(h1)31h\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{- h ^{2} - ( h - 1 ) ^{3} - 1}{h}=limh0h3+2h23hh\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{- h ^{3} + 2 h ^{2} - 3 h}{h} =limh0(h2+2h3)\displaystyle = \lim\limits _{h \rightarrow - 0} {} ( - h ^{2} + 2 h - 3 )=3= - 3 \therefore f(0)=3f' ( 0 ) = - 3 limh+0f(1+h)f(1)h\displaystyle \lim\limits _{h \rightarrow + 0} {} \frac{f ( 1 + h ) - f ( 1 )}{h}=limh+0(1+h)1+h+1+h131h\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{( 1 + h ) | 1 + h | + | 1 + h - 1 | ^{3} - 1}{h} =limh+0(1+h)2+h31h\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{( 1 + h ) ^{2} + | h | ^{3} - 1}{h} =limh+0h3+h2+2hh\displaystyle = \lim\limits _{h \rightarrow + 0} {} \frac{h ^{3} + h ^{2} + 2 h}{h}=limh+0(h2+h+2)\displaystyle = \lim\limits _{h \rightarrow + 0} {} ( h ^{2} + h + 2 )=2= 2 limh0f(1+h)f(1)h\displaystyle \lim\limits _{h \rightarrow - 0} {} \frac{f ( 1 + h ) - f ( 1 )}{h} =limh0(1+h)1+h+1+h131h\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{( 1 + h ) | 1 + h | + | 1 + h - 1 | ^{3} - 1}{h} =limh0(1+h)2+h31h\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{( 1 + h ) ^{2} + | h | ^{3} - 1}{h}=limh0h3+h2+2hh\displaystyle = \lim\limits _{h \rightarrow - 0} {} \frac{- h ^{3} + h ^{2} + 2 h}{h} =limh0(h2+h+2)\displaystyle = \lim\limits _{h \rightarrow - 0} {} ( - h ^{2} + h + 2 )=2= 2 \therefore f(1)=2f' ( 1 ) = 2 \therefore f(0)+f(1)=3+2=1f' ( 0 ) + f' ( 1 ) = - 3 + 2 = - 1 [다른풀이2] f(x)f ( x )={x2+(x1)3(x1)x2(x1)3(0x<1)x2(x1)3(x<0)\displaystyle = {\begin{cases} x ^{2} + ( x - 1 ) ^{3} & ( x \geq 1 ) \\ x ^{2} - ( x - 1 ) ^{3} & ( 0 \leq x < 1 ) \\ - x ^{2} - ( x - 1 ) ^{3} & ( x < 0 ) \end{cases}} 이므로 f(x)={2x+3(x1)2(x>1)2x3(x1)2(0<x<1)2x3(x1)2(x<0)\displaystyle f' ( x ) = {\begin{cases} 2 x + 3 ( x - 1 ) ^{2} & ( x > 1 ) \\ 2 x - 3 ( x - 1 ) ^{2} & ( 0 < x < 1 ) \\ - 2 x - 3 ( x - 1 ) ^{2} & ( x < 0 ) \end{cases}} limx+0f(x)=203(01)2=3\displaystyle \lim\limits _{x \rightarrow + 0} {} f' ( x ) = 2 \cdot 0 - 3 ( 0 - 1 ) ^{2} = - 3 limx0f(x)=203(01)2=3\displaystyle \lim\limits _{x \rightarrow - 0} {} f' ( x ) = - 2 \cdot 0 - 3 ( 0 - 1 ) ^{2} = - 3 \therefore f(0)=3f' ( 0 ) = - 3 limx1+0f(x)=21+3(11)2=2\displaystyle \lim\limits _{x \rightarrow 1 + 0} {} f' ( x ) = 2 \cdot 1 + 3 ( 1 - 1 ) ^{2} = 2 limx10f(x)=213(11)2=2\displaystyle \lim\limits _{x \rightarrow 1 - 0} {} f' ( x ) = 2 \cdot 1 - 3 ( 1 - 1 ) ^{2} = 2 \therefore f(1)=2f' ( 1 ) = 2 이상에서 f(0)+f(1)=3+2=1f' ( 0 ) + f' ( 1 ) = - 3 + 2 = - 1

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