In an ideal turbofan engine shown in the figure below, the compressor is driven by the high pressure turbine, and the fan is driven by the low pressure turbine. The stations 0, 2, 3, 4, 4.5, and 5 refer to free-stream, compressor inlet, compressor outlet, combustor exit, high pressure turbine exit, and low pressure turbine exit, respectively, and the subscript ‘t’ refers to the total condition. Also, τ_r = T_t0/T₀, τ_c = T_t3/T_t2 and τ_λ = T_t4/T₀. The total temperature ratio of the high pressure turbine (T_t4.5/T_t4) is given by ______.