<span>(x+6)^2=(15-x)^2 => х²+12х+36=225-30х+х² => 12х+30х=225-36 => 42х=189.
х=189/42=4,5.
Ответ: х=4,5.
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вложение..................................................
А) 6 см (6 = х)
б) 4 см (12 : 3)
cos(π/7)+cos(π/15)= 2cos((π/7+π/15)/2)*cos((π/7-π/15)/2)= 2cos(11π/105)*cos(4π/105)