QR² = 4² + 12² - 2(4)(12) cos 120
QR² = 16 + 144 - 2(4)(12)(-½)
QR² = 16 + 144 + 48
QR² = 208
QR = √208
QR = 4√13 cm
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QR² = 4² + 12² - 2(4)(12) cos 120
QR² = 16 + 144 - 2(4)(12)(-½)
QR² = 16 + 144 + 48
QR² = 208
QR = √208
QR = 4√13 cm