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Algebra Level 1

( 1 + 2 + 3 + ⋯ + 10 ) − 12 × 3 + 1 2 = x \large \frac{(1+2+3+\cdots+10) - 12 \times 3 + 1 }{2} = x What is x x ?


The answer is 10.

Hopfhen Shane by Hopfhen Shane , 31, Singapore

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3 solutions

( 1 + 2 + 3 + . . . . . . . + 10 ) − 12 × 3 + 1 2 = 11 × 10 2 − 36 + 1 2 = 55 − 36 + 1 2 = 20 2 = 10 = x \dfrac{(1+2+3+.......+10) - 12 \times 3 + 1 }{2}=\dfrac{\frac{11 \times 10}{2}-36+1}{2}=\dfrac{55-36+1}{2}=\dfrac{20}{2}=10=x .

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Shah Faisal
Jul 10, 2017

As digit from 1...10 form a geometric sequence whose sum is 55. 55+1-36÷2=10

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Ashish Menon
Jun 29, 2016

( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 ) − 12 × 3 + 1 2 = 56 − 36 2 = 20 10 = 10 \begin{aligned} \dfrac{(1+2+3+4+5+6+7+8+9+10) - 12×3 + 1}{2} & = \dfrac{56-36}{2}\\ \\ & = \dfrac{20}{10}\\ \\ & = \color{#3D99F6}{\boxed{10}} \end{aligned}

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