Verified answer

1/3*5 = 1/2*( (5 - 3)/3*5) = 1/2*(1/3 - 1/5)

1/5*7 = 1/2*(1/5 - 1/7)

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x/3*5 + x/5*7 + ...... + x/21*23 = x/2(1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/21 - 1/23) = x/2*(1/3 - 1/23) = x/2 * (23 - 3)/69 =,10x/69

10x/69 = 1 11/69

10x/69 = 80/69

x = 8

x² - 64 = 8² - 64 = 0

ответ С