matlab如何定义常数a,利用matlab进行分段拟合求一个常数a，拐点m，直线y=kx+b，数据如下...

x=[3.00221124

2.80819715

2.77570885

2.700689847

2.472327868

2.427454075

2.373975089

2.283402274

2.263844265

2.211565695

2.190535589

2.186051277

2.159868791

2.159868791

2.152924318

2.148267733

2.107786015

2.089391873

2.079441542

2.066862759

2.059238834

2.054123734

2.05284086

2.051556338

2.047692843

2.02287119

2.020222182

2.009555414

2.002830439

2.002830439

1.993338843

1.986503546

1.983756292

1.979621206

1.974081026

1.962907725

1.962907725

1.962907725

1.962907725

1.961502244

1.953027617

1.948763218

1.932969638

1.932969638

1.932969638

1.919859472

1.912501087

1.906575144

1.900613874

1.894616855

1.894616855

1.880990603

1.867176109

1.867176109

1.867176109

1.867176109

1.867176109

1.865629318

1.864080131

1.83736998

1.834180185

1.830980182

1.814824742

1.803358605

1.800058272

1.800058272

1.798404012

1.798404012

1.780024213

1.780024213

1.780024213

1.780024213

1.780024213

1.778336449

1.766441661

1.763017

1.757857918

1.743968805

1.743968805

1.728109442

1.728109442

1.726331664

1.726331664

1.726331664

1.726331664

1.726331664

1.710187816

1.70837786

1.706564623

1.706564623

1.706564623

1.706564623

1.688249093

1.688249093

1.688249093

1.688249093

1.688249093

1.688249093

1.680827909

1.648658626

1.638996715

1.62924054

1.611435915

1.609437912

1.60341984

1.595338988

1.595338988

1.591273942

1.589235205

1.589235205

1.589235205

1.589235205

1.589235205

1.589235205

1.589235205

1.587192303

1.58514522

1.568615918

1.568615918

1.566530411

1.560247668

1.560247668

1.558144618

1.553925203

1.545432582

1.54329811

1.523880024

1.523880024

1.521698998

1.521698998

1.521698998

1.521698998

1.512927012

1.506297154

1.501852702

1.477048724

1.477048724

1.477048724

1.456286733

1.451613827

1.435084525

1.427916036

1.427916036

1.425515074

1.40854497

1.40854497

1.383791231

1.371180723

1.358409158

1.358409158

1.353254507

1.332366019

1.327075001

1.327075001

1.324418957

1.280933845

1.272565596

1.272565596

1.272565596

1.272565596

1.252762968

1.252762968

1.232560261

1.226712291

1.223775432

1.214912744

1.214912744

1.196948189

1.193922468

1.184789985

1.153731588

1.153731588

1.153731588

1.153731588

1.134622726

1.134622726

1.134622726

1.134622726

1.088561953

1.088561953

1.088561953

1.068153081

1.068153081

1.043804052

1.01884732

1.01884732

1.00063188

1.00063188

1.00063188

0.928219303

0.891998039

0.867100488

0.850150929

0.850150929

0.850150929

0.722705983

0.693147181

0.672944473

0.587786665

0.570979547

]

y=[0

0.693147181

1.098612289

1.386294361

1.609437912

1.791759469

1.945910149

2.079441542

2.197224577

2.302585093

2.397895273

2.48490665

2.564949357

2.63905733

2.708050201

2.772588722

2.833213344

2.890371758

2.944438979

2.995732274

3.044522438

3.091042453

3.135494216

3.17805383

3.218875825

3.258096538

3.295836866

3.33220451

3.36729583

3.401197382

3.433987204

3.465735903

3.496507561

3.526360525

3.555348061

3.583518938

3.610917913

3.63758616

3.663561646

3.688879454

3.713572067

3.737669618

3.761200116

3.784189634

3.80666249

3.828641396

3.850147602

3.871201011

3.891820298

3.912023005

3.931825633

3.951243719

3.970291914

3.988984047

4.007333185

4.025351691

4.043051268

4.060443011

4.077537444

4.094344562

4.110873864

4.127134385

4.143134726

4.158883083

4.17438727

4.189654742

4.204692619

4.219507705

4.234106505

4.248495242

4.262679877

4.276666119

4.290459441

4.304065093

4.317488114

4.33073334

4.343805422

4.356708827

4.369447852

4.382026635

4.394449155

4.406719247

4.418840608

4.430816799

4.442651256

4.454347296

4.465908119

4.477336814

4.48863637

4.49980967

4.510859507

4.521788577

4.532599493

4.543294782

4.553876892

4.564348191

4.574710979

4.584967479

4.59511985

4.605170186

4.615120517

4.624972813

4.634728988

4.644390899

4.65396035

4.663439094

4.672828834

4.682131227

4.691347882

4.700480366

4.709530201

4.718498871

4.727387819

4.736198448

4.744932128

4.753590191

4.762173935

4.770684624

4.779123493

4.787491743

4.795790546

4.804021045

4.812184355

4.820281566

4.828313737

4.836281907

4.844187086

4.852030264

4.859812404

4.86753445

4.875197323

4.882801923

4.890349128

4.8978398

4.905274778

4.912654886

4.919980926

4.927253685

4.934473933

4.941642423

4.94875989

4.955827058

4.96284463

4.9698133

4.976733742

4.983606622

4.990432587

4.997212274

5.003946306

5.010635294

5.017279837

5.023880521

5.030437921

5.036952602

5.043425117

5.049856007

5.056245805

5.062595033

5.068904202

5.075173815

5.081404365

5.087596335

5.093750201

5.099866428

5.105945474

5.111987788

5.117993812

5.123963979

5.129898715

5.135798437

5.141663557

5.147494477

5.153291594

5.159055299

5.164785974

5.170483995

5.176149733

5.18178355

5.187385806

5.192956851

5.198497031

5.204006687

5.209486153

5.214935758

5.220355825

5.225746674

5.231108617

5.236441963

5.241747015

5.247024072

5.252273428

5.257495372

5.262690189

5.267858159

5.272999559

5.278114659

5.283203729

5.288267031

5.293304825

5.298317367

]