50. S = 2ab + 2ac + 2bc
Если a=5 , b=8, S = 210, то
210 = 2*5*8 + 2*5*с + 2*8*с
210 = 80 + 10с + 16с
26с = 130
с = 5
51 a*b = |a||b| cos α = 2 * 5 * cos π/6 = 10 * √3/2 = 5√3
52 a) a*b = |a||b| cos α = 3 *1 * cos 45 = 3 * √2/2 = 3/2 √2
b) a*b = |a||b| cos α = 3 * 2 * cos π/3 = 6 * 1/2 = 3