An algebra problem by anshu garg

Algebra Level 3

Find the minimum value of the polynomial 2 x 2 3 x + 4 2x^2-3x+4 for real x x .

45/8 23/8 25/8 6/8

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2 solutions

Zee Ell
Nov 12, 2016

The minimum value can be easily determined by completing the square:

2 x 2 3 x + 4 = 2 ( x 2 3 2 x ) + 4 = 2 ( x 3 4 ) 2 2 × ( 3 4 ) 2 + 4 = 2x^2 - 3x + 4 = 2( x^2 - \frac {3}{2} x ) + 4 = 2(x - \frac {3}{4} )^2 - 2 × ( \frac {3}{4} )^2 + 4 =

= 2 ( x 3 4 ) 2 9 8 + 4 = 2 ( x 3 4 ) 2 + 23 8 = 2(x - \frac {3}{4} )^2 - \frac {9}{8} + 4 = 2(x - \frac {3}{4} )^2 + \frac {23}{8}

The latter expression is minimal, when the square is zero ( x = 0.75 ).

Hence, the minimum value is:

23 8 \boxed { \frac {23}{8} }

Good idea but you can use direct formula which is derived using graph or by completing the square

anshu garg - 4 years, 6 months ago
Anshu Garg
Nov 12, 2016

The minimum value is given by D 4 a \frac{-D}{4a} using the graph

= 4 a c b 2 4 a \frac{4ac-b^2}{4a}

= 32 9 8 \frac{32-9}{8}

= 23 8 \frac{23}{8}

Use formula for vertex of parabola of polynomial

Lakshay Rana - 4 years, 2 months ago

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