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math

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Natasha has to determain the number of toothpicks in the 15th step, step one has four toothpicks step two has 12 toothpicks and step three has 24 toothpicks. She does not want to draw 15 pictures then count the toothpicks. Explain or show how she could do this.

  • math -

    tn = a1 + (n-1)d

    t2 = 12 and t3 = 24 so d= 24-13=11

    so we need to find a1

    t2=a1 + (n-1)d
    12 = a1 + (2-1)11
    12 = a1 +11
    1 = a1

    then

    t15 = 1 + (15-1)11
    t15 = 1 + 154
    t15 = 155

  • math -

    use the arithmetic sequence formula
    a(n)=a(1)+(n-1)d
    last=first+(number-1)difference
    ?=4+(15-1)20
    ?=284

    im pretty sure lol

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