PR = QR ------>
L Q = L QPR = (180 - L PRQ)\2 = (180 - 120)\2 = 30 град. =>
L P = 90 - 30 = 60 град
Треугольник SPR:
L S = 90 град.
L SPR = L P - L QPR = 60 - 30 = 30 град. =>
PR = 2*SP ----> SR = x -----> PR =2x
PS^2 + SR^2 = PR^2 или 7^2 + x^2 = (2x)^2 или
49 = 3x^2 ------> x = SP = 7V3\3 и PR = 2x = 2*7V3\3 = 14V3