Home > Math Group > Question and Answer

Math Group

Question:


lesliee
Student
High School - 10th Grade

 Evalute 2x3 - 3x2 + 4x + 5 with x = 4.

Dang. Please help. I suck at algebra.

Rate question:

Posted by lesliee on Wednesday November 26, 2008 at 7:07 PM and tagged with algebra, math.

Answers:


  1. lynn30k Teacher
    High School - 10th Grade

    eNotes Editor

    You'll be fine.  Just keep the order you should do the operations in straight (PEMDAS-Please excuse my dear Aunt Sally, to help you remember; parentheses, exponents, multiplication, division, addition, subtraction) So for the first problem plug in "4" where you have an "X".  You get:

    2x3 - 3x2 + 4x4 + 5

    You only have mult., addition, and subtraction, so....

    6-6+16+5, then 6-27, or -21 for an answer.

    Good luck, you can do it!

    Rate answer:

    Posted by lynn30k on Wednesday November 26, 2008 at 8:15 PM


  2. enotechris Teacher
    Vocational

    eNotes Editor

    Don't suck.  Just practice.  It's one of these stupid things in math that "x" is used as a variable AND as the multiplication sign.  So let's clarify each problem, with "*" meaning multiply:

    1a.  2*3 - 3*2 + 4x + 5  where x=4

     OK.  This is the classic "Which operation do I perform first?" problem.  Consider the first part of the equation, 2*3 - 3*2.  If you do it in order, you get 6-3, which leaves 3, times 2, which leaves you with 6.  This is a trap!!  Remember the order of operations: PEMDAS (Parenthesis, Exponents, Multiplication, Division, Addition, Subtraction, done left to right.)  The other problem is to keep track of whether the + and - refers to a binary operation (add,subtract) or is the sign (pos,neg) of the integer! So you would clarify the problem by putting parenthesis around the multiplication portions and the negative integers like:

    1b.   (2*3)-(3*2) + 4x + 5  where x=4  (no neg. integers here)

       subsituting 4 for x gives:

    1c.   (2*3) - (3*2) + (4*4) + 5

      so you're left with:

    1d.   6 - 6 + 16 + 5  

      or

    1e.  6 - (6 + 16 + 5) (...remember addition before subtraction!)

      which gives you:

    1f.   6 - 27

    1g.   -21

    Rate answer:

    Posted by enotechris on Wednesday November 26, 2008 at 8:39 PM

  3. hala718
    hala718 Teacher

    eNotes Editor

    Actually, even though I agree with the PEDMAS, However, we tend to use it ina wrong way.

    PEDMAS is used to simplify the equations for beginners but the fact is that subtractions and addition have the same priority as well as multiplication and division have the same priority. For example

    2*4/5 = ?

    lets do the multiplication first

    then, 2*4/5 = 8/5 = 1.6

    or 2* 0.8 = 1.6 which is the same answer.

    so the answer for your question is as follow

    2*3 - 3*2 + 4X + 5 =

    6 - 6 + 16 + 5 =

    6 + (-6) + 16 + 5 = 21 and not -21 because the ( - ) only applies for what comes next ,if there is no parenthesis, which is the 6.

    Hope that helps.

    Rate answer:

    Posted by hala718 on Monday December 1, 2008 at 3:30 PM

  4. dracomath
    dracomath Student
    High School - 11th Grade

    Substitute x with 4.

    2x³-3x²+4x+5

    2(4)³-3(4)²+4(4)+5
    2(64)-48+16+5

    101

    Rate answer:

    Posted by dracomath on Sunday December 28, 2008 at 8:36 AM