x²/4+y²/7=1,设M(2cosθ,√7sinθ)d=|6cosθ-2√7sinθ-16|/√136cosθ-2√7sinθ-16=8(cosθ*3/4-√7sinθ/4)-16=8cos(θ+α)-16θ+α=2kπ,k∈Z时,取得最小值.sinα=√7/4,cosα=3/4.sinθ=sin(2kπ-α)...sinα=√7/4，cosα=3/4这一步怎么来?8（cosθ*3/4-√7sinθ/4)-16=8cos(θ+α)-16令cosα=3/4，sinα=√7/4才有 8cos(θ+α)-16