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April 28th, 2014, 12:14 PM   #31
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Quote:
Originally Posted by XPMai View Post

Please teach me how to solve this question:

Find the value of: 7+9+11+13+15+17+……+55+57+59+61

Ans: 952

Thanks!
$\displaystyle \ \color{blue} {7 = 7+2\cdot0\\\;\\ \ 9 = 7+2\cdot1\\\;\\11 = 7+2\cdot2\\\;\\13 = 7+2\cdot3\\\vdots\\61=7+2\cdot27\\\;\\7+9+11+13+ ... +61 = 7\cdot28+2(1+2+3+ ... + 27) = 7\cdot28+2\cdot\dfrac{27\cdot28}{2}=7\cdot28+27 \cdot 28 =28(7 + 27) = 28 \cdot 34 = ...}$
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